diff --git a/src/ccstatusline.ts b/src/ccstatusline.ts
index 6998ca3..e2a2bf1 100644
--- a/src/ccstatusline.ts
+++ b/src/ccstatusline.ts
@@ -15,7 +15,7 @@ import {
     saveSettings
 } from './utils/config';
 import {
-    getBlockMetrics,
+    getCachedBlockMetrics,
     getSessionDuration,
     getTokenMetrics
 } from './utils/jsonl';
@@ -100,7 +100,7 @@ async function renderMultipleLines(data: StatusJSON) {
 
     let blockMetrics: BlockMetrics | null = null;
     if (hasBlockTimer) {
-        blockMetrics = getBlockMetrics();
+        blockMetrics = getCachedBlockMetrics();
     }
 
     // Create render context
diff --git a/src/utils/__tests__/block-cache.test.ts b/src/utils/__tests__/block-cache.test.ts
new file mode 100644
index 0000000..838ab12
--- /dev/null
+++ b/src/utils/__tests__/block-cache.test.ts
@@ -0,0 +1,291 @@
+import * as fs from 'fs';
+import { createHash } from 'node:crypto';
+import os from 'node:os';
+import path from 'node:path';
+import {
+    afterEach,
+    beforeEach,
+    describe,
+    expect,
+    it,
+    vi
+} from 'vitest';
+
+import {
+    getBlockCachePath,
+    readBlockCache,
+    writeBlockCache
+} from '../jsonl';
+
+function getExpectedCachePath(homeDir: string, configDir: string): string {
+    const normalizedConfigDir = path.resolve(configDir);
+    const configHash = createHash('sha256')
+        .update(normalizedConfigDir)
+        .digest('hex')
+        .slice(0, 16);
+
+    return path.join(homeDir, '.cache', 'ccstatusline', `block-cache-${configHash}.json`);
+}
+
+describe('Block Cache Functions', () => {
+    let tempDir: string;
+    let originalClaudeConfigDir: string | undefined;
+
+    beforeEach(() => {
+        // Create a temp directory for test isolation
+        tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'ccstatusline-test-'));
+        // Mock os.homedir to use temp directory
+        vi.spyOn(os, 'homedir').mockReturnValue(tempDir);
+        originalClaudeConfigDir = process.env.CLAUDE_CONFIG_DIR;
+        process.env.CLAUDE_CONFIG_DIR = path.join(tempDir, '.claude-default');
+    });
+
+    afterEach(() => {
+        vi.restoreAllMocks();
+        if (originalClaudeConfigDir === undefined) {
+            delete process.env.CLAUDE_CONFIG_DIR;
+        } else {
+            process.env.CLAUDE_CONFIG_DIR = originalClaudeConfigDir;
+        }
+        // Clean up temp directory
+        fs.rmSync(tempDir, { recursive: true, force: true });
+    });
+
+    describe('getBlockCachePath', () => {
+        it('should return the correct cache path', () => {
+            const cachePath = getBlockCachePath();
+            expect(cachePath).toBe(getExpectedCachePath(tempDir, path.join(tempDir, '.claude-default')));
+        });
+
+        it('should return different cache paths for different config directories', () => {
+            const profileA = path.join(tempDir, '.claude-profile-a');
+            const profileB = path.join(tempDir, '.claude-profile-b');
+
+            const pathA = getBlockCachePath(profileA);
+            const pathB = getBlockCachePath(profileB);
+
+            expect(pathA).toBe(getExpectedCachePath(tempDir, profileA));
+            expect(pathB).toBe(getExpectedCachePath(tempDir, profileB));
+            expect(pathA).not.toBe(pathB);
+        });
+    });
+
+    describe('readBlockCache', () => {
+        it('should return null when cache file does not exist', () => {
+            const result = readBlockCache();
+            expect(result).toBeNull();
+        });
+
+        it('should return cached date when cache file is valid', () => {
+            const testDate = new Date('2025-01-26T14:00:00.000Z');
+            const cachePath = getBlockCachePath();
+            const cacheDir = path.dirname(cachePath);
+            fs.mkdirSync(cacheDir, { recursive: true });
+            fs.writeFileSync(cachePath, JSON.stringify({ startTime: testDate.toISOString() }));
+
+            const result = readBlockCache();
+            expect(result).toEqual(testDate);
+        });
+
+        it('should return cached date when configDir matches expected value', () => {
+            const testDate = new Date('2025-01-26T14:00:00.000Z');
+            const configDir = path.join(tempDir, '.claude-profile-a');
+            const cachePath = getBlockCachePath(configDir);
+            const cacheDir = path.dirname(cachePath);
+            fs.mkdirSync(cacheDir, { recursive: true });
+            fs.writeFileSync(
+                cachePath,
+                JSON.stringify({ startTime: testDate.toISOString(), configDir })
+            );
+
+            const result = readBlockCache(configDir);
+            expect(result).toEqual(testDate);
+        });
+
+        it('should return null when cache configDir does not match expected value', () => {
+            const testDate = new Date('2025-01-26T14:00:00.000Z');
+            const expectedConfigDir = path.join(tempDir, '.claude-profile-b');
+            const cachePath = getBlockCachePath(expectedConfigDir);
+            const cacheDir = path.dirname(cachePath);
+            fs.mkdirSync(cacheDir, { recursive: true });
+            fs.writeFileSync(
+                cachePath,
+                JSON.stringify({
+                    startTime: testDate.toISOString(),
+                    configDir: path.join(tempDir, '.claude-profile-a')
+                })
+            );
+
+            const result = readBlockCache(expectedConfigDir);
+            expect(result).toBeNull();
+        });
+
+        it('should return null when expected configDir is provided but cache has legacy schema', () => {
+            const testDate = new Date('2025-01-26T14:00:00.000Z');
+            const expectedConfigDir = path.join(tempDir, '.claude-profile-a');
+            const cachePath = getBlockCachePath(expectedConfigDir);
+            const cacheDir = path.dirname(cachePath);
+            fs.mkdirSync(cacheDir, { recursive: true });
+            fs.writeFileSync(cachePath, JSON.stringify({ startTime: testDate.toISOString() }));
+
+            const result = readBlockCache(expectedConfigDir);
+            expect(result).toBeNull();
+        });
+
+        it('should return null when cache file has invalid JSON', () => {
+            const cachePath = getBlockCachePath();
+            const cacheDir = path.dirname(cachePath);
+            fs.mkdirSync(cacheDir, { recursive: true });
+            fs.writeFileSync(cachePath, 'not valid json');
+
+            const result = readBlockCache();
+            expect(result).toBeNull();
+        });
+
+        it('should return null when cache file has missing startTime', () => {
+            const cachePath = getBlockCachePath();
+            const cacheDir = path.dirname(cachePath);
+            fs.mkdirSync(cacheDir, { recursive: true });
+            fs.writeFileSync(cachePath, JSON.stringify({}));
+
+            const result = readBlockCache();
+            expect(result).toBeNull();
+        });
+
+        it('should return null when startTime is not a string', () => {
+            const cachePath = getBlockCachePath();
+            const cacheDir = path.dirname(cachePath);
+            fs.mkdirSync(cacheDir, { recursive: true });
+            fs.writeFileSync(cachePath, JSON.stringify({ startTime: 12345 }));
+
+            const result = readBlockCache();
+            expect(result).toBeNull();
+        });
+
+        it('should return null when startTime is invalid date string', () => {
+            const cachePath = getBlockCachePath();
+            const cacheDir = path.dirname(cachePath);
+            fs.mkdirSync(cacheDir, { recursive: true });
+            fs.writeFileSync(cachePath, JSON.stringify({ startTime: 'not a date' }));
+
+            const result = readBlockCache();
+            expect(result).toBeNull();
+        });
+    });
+
+    describe('writeBlockCache', () => {
+        it('should create directory and write cache file', () => {
+            const testDate = new Date('2025-01-26T14:00:00.000Z');
+            const configDir = path.join(tempDir, '.claude-profile-a');
+
+            writeBlockCache(testDate, configDir);
+
+            const cachePath = getBlockCachePath(configDir);
+            expect(fs.existsSync(cachePath)).toBe(true);
+            const content = fs.readFileSync(cachePath, 'utf-8');
+            const parsed = JSON.parse(content) as { startTime: string; configDir: string };
+            expect(parsed.startTime).toBe(testDate.toISOString());
+            expect(parsed.configDir).toBe(path.resolve(configDir));
+        });
+
+        it('should overwrite existing cache file', () => {
+            const firstDate = new Date('2025-01-26T14:00:00.000Z');
+            const secondDate = new Date('2025-01-26T16:00:00.000Z');
+            const configDir = path.join(tempDir, '.claude-profile-a');
+
+            writeBlockCache(firstDate, configDir);
+            writeBlockCache(secondDate, configDir);
+
+            const cachePath = getBlockCachePath(configDir);
+            const content = fs.readFileSync(cachePath, 'utf-8');
+            const parsed = JSON.parse(content) as { startTime: string; configDir: string };
+            expect(parsed.startTime).toBe(secondDate.toISOString());
+            expect(parsed.configDir).toBe(path.resolve(configDir));
+        });
+    });
+});
+
+describe('getCachedBlockMetrics integration', () => {
+    let tempDir: string;
+    let originalClaudeConfigDir: string | undefined;
+
+    beforeEach(() => {
+        tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'ccstatusline-test-'));
+        vi.spyOn(os, 'homedir').mockReturnValue(tempDir);
+        // Mock CLAUDE_CONFIG_DIR to point to a non-existent directory
+        // This ensures getBlockMetrics returns null when cache is expired
+        originalClaudeConfigDir = process.env.CLAUDE_CONFIG_DIR;
+        process.env.CLAUDE_CONFIG_DIR = path.join(tempDir, '.claude-nonexistent');
+    });
+
+    afterEach(() => {
+        vi.restoreAllMocks();
+        // Restore original CLAUDE_CONFIG_DIR
+        if (originalClaudeConfigDir === undefined) {
+            delete process.env.CLAUDE_CONFIG_DIR;
+        } else {
+            process.env.CLAUDE_CONFIG_DIR = originalClaudeConfigDir;
+        }
+        fs.rmSync(tempDir, { recursive: true, force: true });
+    });
+
+    it('should return cached result when cache is within block duration', async () => {
+        const { getCachedBlockMetrics, writeBlockCache, getBlockCachePath } = await import('../jsonl');
+
+        // Set up a cache with a start time 2 hours ago (within 5-hour block)
+        const testStartTime = new Date();
+        testStartTime.setHours(testStartTime.getHours() - 2);
+
+        writeBlockCache(testStartTime);
+
+        // Verify cache was written
+        expect(fs.existsSync(getBlockCachePath())).toBe(true);
+
+        const result = getCachedBlockMetrics();
+
+        expect(result).not.toBeNull();
+        expect(result?.startTime.getTime()).toBe(testStartTime.getTime());
+    });
+
+    it('should recalculate when cache is expired (beyond block duration)', async () => {
+        const { getCachedBlockMetrics, writeBlockCache } = await import('../jsonl');
+
+        // Set up a cache with a start time 6 hours ago (beyond 5-hour block)
+        const expiredStartTime = new Date();
+        expiredStartTime.setHours(expiredStartTime.getHours() - 6);
+
+        writeBlockCache(expiredStartTime);
+
+        const result = getCachedBlockMetrics();
+
+        // Should return null because cache is expired and no real JSONL files exist
+        expect(result).toBeNull();
+    });
+
+    it('should recalculate when no cache exists', async () => {
+        const { getCachedBlockMetrics } = await import('../jsonl');
+
+        const result = getCachedBlockMetrics();
+
+        // Should return null because no cache and no real JSONL files exist
+        expect(result).toBeNull();
+    });
+
+    it('should recalculate when cache belongs to a different config directory', async () => {
+        const { getCachedBlockMetrics, writeBlockCache } = await import('../jsonl');
+        const profileA = path.join(tempDir, '.claude-profile-a');
+        const profileB = path.join(tempDir, '.claude-profile-b');
+
+        const testStartTime = new Date();
+        testStartTime.setHours(testStartTime.getHours() - 2);
+
+        process.env.CLAUDE_CONFIG_DIR = profileA;
+        writeBlockCache(testStartTime);
+
+        process.env.CLAUDE_CONFIG_DIR = profileB;
+        const result = getCachedBlockMetrics();
+
+        // Should return null because cache is fresh but scoped to a different profile
+        expect(result).toBeNull();
+    });
+});
\ No newline at end of file
diff --git a/src/utils/jsonl.ts b/src/utils/jsonl.ts
index d00923c..6d35a1d 100644
--- a/src/utils/jsonl.ts
+++ b/src/utils/jsonl.ts
@@ -1,4 +1,6 @@
 import * as fs from 'fs';
+import { createHash } from 'node:crypto';
+import os from 'node:os';
 import path from 'node:path';
 import { globSync } from 'tinyglobby';
 import { promisify } from 'util';
@@ -15,6 +17,130 @@ import { getClaudeConfigDir } from './claude-settings';
 const readFile = promisify(fs.readFile);
 const readFileSync = fs.readFileSync;
 const statSync = fs.statSync;
+const writeFileSync = fs.writeFileSync;
+const mkdirSync = fs.mkdirSync;
+const existsSync = fs.existsSync;
+
+// --- Block Cache Functions ---
+
+interface BlockCache {
+    startTime: string;
+    configDir?: string;
+}
+
+function normalizeConfigDir(configDir: string): string {
+    return path.resolve(configDir);
+}
+
+/**
+ * Returns the path to the block cache file for a specific Claude config directory
+ */
+export function getBlockCachePath(configDir = getClaudeConfigDir()): string {
+    const normalizedConfigDir = normalizeConfigDir(configDir);
+    const configHash = createHash('sha256')
+        .update(normalizedConfigDir)
+        .digest('hex')
+        .slice(0, 16);
+
+    return path.join(
+        os.homedir(),
+        '.cache',
+        'ccstatusline',
+        `block-cache-${configHash}.json`
+    );
+}
+
+/**
+ * Reads the block cache file and returns the cached start time
+ * Returns null if cache doesn't exist or is invalid
+ */
+export function readBlockCache(expectedConfigDir?: string): Date | null {
+    try {
+        const normalizedExpectedConfigDir = expectedConfigDir !== undefined
+            ? normalizeConfigDir(expectedConfigDir)
+            : undefined;
+        const cachePath = getBlockCachePath(normalizedExpectedConfigDir);
+        if (!existsSync(cachePath)) {
+            return null;
+        }
+        const content = readFileSync(cachePath, 'utf-8');
+        const cache = JSON.parse(content) as BlockCache;
+        if (typeof cache.startTime !== 'string') {
+            return null;
+        }
+        if (normalizedExpectedConfigDir !== undefined) {
+            if (typeof cache.configDir !== 'string') {
+                return null;
+            }
+            if (cache.configDir !== normalizedExpectedConfigDir) {
+                return null;
+            }
+        }
+        const date = new Date(cache.startTime);
+        if (Number.isNaN(date.getTime())) {
+            return null;
+        }
+        return date;
+    } catch {
+        return null;
+    }
+}
+
+/**
+ * Writes the block start time to the cache file
+ * Creates the cache directory if it doesn't exist
+ */
+export function writeBlockCache(startTime: Date, configDir = getClaudeConfigDir()): void {
+    try {
+        const normalizedConfigDir = normalizeConfigDir(configDir);
+        const cachePath = getBlockCachePath(normalizedConfigDir);
+        const cacheDir = path.dirname(cachePath);
+        if (!existsSync(cacheDir)) {
+            mkdirSync(cacheDir, { recursive: true });
+        }
+        const cache: BlockCache = {
+            startTime: startTime.toISOString(),
+            configDir: normalizedConfigDir
+        };
+        writeFileSync(cachePath, JSON.stringify(cache), 'utf-8');
+    } catch {
+        // Silently fail - caching is best-effort
+    }
+}
+
+/**
+ * Gets block metrics with caching support
+ * Returns cached result if still valid, otherwise recalculates
+ */
+export function getCachedBlockMetrics(sessionDurationHours = 5): BlockMetrics | null {
+    const sessionDurationMs = sessionDurationHours * 60 * 60 * 1000;
+    const now = new Date();
+    const activeConfigDir = getClaudeConfigDir();
+
+    // Check cache first
+    const cachedStartTime = readBlockCache(activeConfigDir);
+    if (cachedStartTime) {
+        const blockEndTime = new Date(cachedStartTime.getTime() + sessionDurationMs);
+        if (now.getTime() <= blockEndTime.getTime()) {
+            // Cache is valid - return cached result
+            return {
+                startTime: cachedStartTime,
+                lastActivity: now // We don't cache lastActivity, use current time
+            };
+        }
+        // Cache expired - need to recalculate
+    }
+
+    // Cache miss or expired - run full calculation
+    const metrics = getBlockMetrics();
+
+    // Write to cache if we found a valid block
+    if (metrics) {
+        writeBlockCache(metrics.startTime, activeConfigDir);
+    }
+
+    return metrics;
+}
 
 export async function getSessionDuration(transcriptPath: string): Promise<string | null> {
     try {