diff --git a/src/collector.ts b/src/collector.ts
index 1cfc447..e4241c0 100644
--- a/src/collector.ts
+++ b/src/collector.ts
@@ -1,7 +1,7 @@
 // Data collector - reads Codex CLI storage and returns raw data
 
 import { createReadStream } from "node:fs";
-import { readdir, readFile, stat } from "node:fs/promises";
+import { readdir, stat } from "node:fs/promises";
 import { join } from "node:path";
 import os from "node:os";
 import { createInterface } from "node:readline";
@@ -80,18 +80,26 @@ export async function listCodexSessionFiles(year: number): Promise<string[]> {
 
 export async function getCodexFirstPromptTimestamp(): Promise<number | null> {
   try {
-    const raw = await readFile(CODEX_HISTORY_PATH, "utf8");
     let minTs: number | null = null;
-    for (const line of raw.split("\n")) {
-      if (!line.trim()) continue;
-      try {
-        const entry = JSON.parse(line) as { ts?: number };
-        if (!entry.ts) continue;
-        if (minTs === null || entry.ts < minTs) {
-          minTs = entry.ts;
-        }
-      } catch {
-        // Skip malformed lines
+
+    // Stream rather than `readFile` to avoid loading large history files into memory.
+    const rl = createInterface({
+      input: createReadStream(CODEX_HISTORY_PATH),
+      crlfDelay: Infinity,
+    });
+
+    for await (const line of rl) {
+      const trimmed = line.trim();
+      if (!trimmed) continue;
+
+      // Avoid JSON.parse in the hot path; we only need `ts`.
+      const match = trimmed.match(/"ts"\s*:\s*(\d+)/);
+      if (!match) continue;
+      const ts = Number(match[1]);
+      if (!Number.isFinite(ts) || ts <= 0) continue;
+
+      if (minTs === null || ts < minTs) {
+        minTs = ts;
       }
     }
     return minTs;
@@ -102,140 +110,23 @@ export async function getCodexFirstPromptTimestamp(): Promise<number | null> {
 
 export async function collectCodexUsageData(year: number): Promise<CodexUsageData> {
   const files = await listCodexSessionFiles(year);
+  const concurrency = Math.max(1, Math.min(os.cpus()?.length ?? 4, 8));
+  const results = await asyncPool(concurrency, files, processSessionFile);
+
   const events: CodexUsageEvent[] = [];
   const dailyActivity = new Map<string, number>();
   const projects = new Set<string>();
   let totalMessages = 0;
   let earliestSessionDate: Date | null = null;
 
-  for (const filePath of files) {
-    let previousTotals: RawUsage | null = null;
-    let currentModel: string | undefined;
-    let currentModelIsFallback = false;
-    let legacyFallbackUsed = false;
-
-    const rl = createInterface({
-      input: createReadStream(filePath),
-      crlfDelay: Infinity,
-    });
-
-    for await (const line of rl) {
-      const trimmed = line.trim();
-      if (!trimmed) continue;
-      let entry: any;
-      try {
-        entry = JSON.parse(trimmed);
-      } catch {
-        continue;
-      }
-
-      const entryType = entry?.type;
-
-      if (entryType === "session_meta") {
-        const sessionTimestamp = entry?.payload?.timestamp ?? entry?.timestamp;
-        if (sessionTimestamp) {
-          const sessionDate = new Date(sessionTimestamp);
-          if (!earliestSessionDate || sessionDate < earliestSessionDate) {
-            earliestSessionDate = sessionDate;
-          }
-        }
-        const cwd = entry?.payload?.cwd;
-        if (cwd) {
-          projects.add(cwd);
-        }
-        continue;
-      }
-
-      if (entryType === "turn_context") {
-        const model = extractModel(entry?.payload);
-        if (model) {
-          currentModel = model;
-          currentModelIsFallback = false;
-        }
-        continue;
-      }
-
-      if (entryType === "event_msg") {
-        const payload = entry?.payload;
-        if (payload?.type === "user_message") {
-          totalMessages += 1;
-          const timestamp = entry?.timestamp;
-          if (timestamp) {
-            const dateKey = formatDateKey(new Date(timestamp));
-            dailyActivity.set(dateKey, (dailyActivity.get(dateKey) || 0) + 1);
-          }
-          continue;
-        }
-
-        if (payload?.type !== "token_count") {
-          continue;
-        }
-
-        const timestamp = entry?.timestamp;
-        if (!timestamp) continue;
-
-        const info = payload?.info;
-        const lastUsage = normalizeRawUsage(info?.last_token_usage);
-        const totalUsage = normalizeRawUsage(info?.total_token_usage);
-
-        let raw = lastUsage;
-        if (!raw && totalUsage) {
-          raw = subtractRawUsage(totalUsage, previousTotals);
-        }
-
-        if (totalUsage) {
-          previousTotals = totalUsage;
-        }
-
-        if (!raw) continue;
-
-        const delta = convertToDelta(raw);
-        if (
-          delta.inputTokens === 0 &&
-          delta.cachedInputTokens === 0 &&
-          delta.outputTokens === 0 &&
-          delta.reasoningOutputTokens === 0
-        ) {
-          continue;
-        }
-
-        const extractedModel = extractModel({ ...payload, info });
-        let isFallback = false;
-        if (extractedModel) {
-          currentModel = extractedModel;
-          currentModelIsFallback = false;
-        }
-
-        let model = extractedModel ?? currentModel;
-        if (!model) {
-          model = LEGACY_FALLBACK_MODEL;
-          isFallback = true;
-          legacyFallbackUsed = true;
-          currentModel = model;
-          currentModelIsFallback = true;
-        } else if (!extractedModel && currentModelIsFallback) {
-          isFallback = true;
-        }
-
-        events.push({
-          timestamp,
-          model,
-          inputTokens: delta.inputTokens,
-          cachedInputTokens: delta.cachedInputTokens,
-          outputTokens: delta.outputTokens,
-          reasoningOutputTokens: delta.reasoningOutputTokens,
-          totalTokens: delta.totalTokens,
-        });
-
-        if (isFallback) {
-          // No-op for now; kept for parity with ccusage
-        }
-      }
-    }
-
-    if (legacyFallbackUsed) {
-      // ignore - best-effort
+  for (const res of results) {
+    totalMessages += res.totalMessages;
+    if (res.earliestSessionDate && (!earliestSessionDate || res.earliestSessionDate < earliestSessionDate)) {
+      earliestSessionDate = res.earliestSessionDate;
     }
+    for (const project of res.projects) projects.add(project);
+    mergeCountMap(dailyActivity, res.dailyActivity);
+    if (res.events.length > 0) events.push(...res.events);
   }
 
   events.sort((a, b) => new Date(a.timestamp).getTime() - new Date(b.timestamp).getTime());
@@ -339,6 +230,202 @@ function asNonEmptyString(value: unknown): string | undefined {
   return trimmed === "" ? undefined : trimmed;
 }
 
+type FileUsageResult = {
+  events: CodexUsageEvent[];
+  dailyActivity: Map<string, number>;
+  totalMessages: number;
+  projects: Set<string>;
+  earliestSessionDate: Date | null;
+};
+
+async function processSessionFile(filePath: string): Promise<FileUsageResult> {
+  const events: CodexUsageEvent[] = [];
+  const dailyActivity = new Map<string, number>();
+  const projects = new Set<string>();
+  let totalMessages = 0;
+  let earliestSessionDate: Date | null = null;
+
+  let previousTotals: RawUsage | null = null;
+  let currentModel: string | undefined;
+  let currentModelIsFallback = false;
+  let legacyFallbackUsed = false;
+
+  const rl = createInterface({
+    input: createReadStream(filePath),
+    crlfDelay: Infinity,
+  });
+
+  for await (const line of rl) {
+    const trimmed = line.trim();
+    if (!trimmed) continue;
+
+    // Fast path: avoid JSON.parse for irrelevant lines (most lines are `response_item` etc.).
+    const entryType = getTopLevelType(trimmed);
+    if (entryType !== "session_meta" && entryType !== "turn_context" && entryType !== "event_msg") {
+      continue;
+    }
+
+    let entry: any;
+    try {
+      entry = JSON.parse(trimmed);
+    } catch {
+      continue;
+    }
+
+    if (entryType === "session_meta") {
+      const sessionTimestamp = entry?.payload?.timestamp ?? entry?.timestamp;
+      if (sessionTimestamp) {
+        const sessionDate = new Date(sessionTimestamp);
+        if (!earliestSessionDate || sessionDate < earliestSessionDate) {
+          earliestSessionDate = sessionDate;
+        }
+      }
+      const cwd = entry?.payload?.cwd;
+      if (cwd) {
+        projects.add(cwd);
+      }
+      continue;
+    }
+
+    if (entryType === "turn_context") {
+      const model = extractModel(entry?.payload);
+      if (model) {
+        currentModel = model;
+        currentModelIsFallback = false;
+      }
+      continue;
+    }
+
+    if (entryType === "event_msg") {
+      const payload = entry?.payload;
+      if (payload?.type === "user_message") {
+        totalMessages += 1;
+        const timestamp = entry?.timestamp;
+        if (timestamp) {
+          const dateKey = formatDateKey(new Date(timestamp));
+          dailyActivity.set(dateKey, (dailyActivity.get(dateKey) || 0) + 1);
+        }
+        continue;
+      }
+
+      if (payload?.type !== "token_count") {
+        continue;
+      }
+
+      const timestamp = entry?.timestamp;
+      if (!timestamp) continue;
+
+      const info = payload?.info;
+      const lastUsage = normalizeRawUsage(info?.last_token_usage);
+      const totalUsage = normalizeRawUsage(info?.total_token_usage);
+
+      let raw = lastUsage;
+      if (!raw && totalUsage) {
+        raw = subtractRawUsage(totalUsage, previousTotals);
+      }
+
+      if (totalUsage) {
+        previousTotals = totalUsage;
+      }
+
+      if (!raw) continue;
+
+      const delta = convertToDelta(raw);
+      if (
+        delta.inputTokens === 0 &&
+        delta.cachedInputTokens === 0 &&
+        delta.outputTokens === 0 &&
+        delta.reasoningOutputTokens === 0
+      ) {
+        continue;
+      }
+
+      // `info` is already part of the payload; avoid object spreading on the hot path.
+      const extractedModel = extractModel(payload);
+      let isFallback = false;
+      if (extractedModel) {
+        currentModel = extractedModel;
+        currentModelIsFallback = false;
+      }
+
+      let model = extractedModel ?? currentModel;
+      if (!model) {
+        model = LEGACY_FALLBACK_MODEL;
+        isFallback = true;
+        legacyFallbackUsed = true;
+        currentModel = model;
+        currentModelIsFallback = true;
+      } else if (!extractedModel && currentModelIsFallback) {
+        isFallback = true;
+      }
+
+      events.push({
+        timestamp,
+        model,
+        inputTokens: delta.inputTokens,
+        cachedInputTokens: delta.cachedInputTokens,
+        outputTokens: delta.outputTokens,
+        reasoningOutputTokens: delta.reasoningOutputTokens,
+        totalTokens: delta.totalTokens,
+      });
+
+      if (isFallback) {
+        // No-op for now; kept for parity with ccusage
+      }
+    }
+  }
+
+  if (legacyFallbackUsed) {
+    // ignore - best-effort
+  }
+
+  return { events, dailyActivity, totalMessages, projects, earliestSessionDate };
+}
+
+function mergeCountMap(into: Map<string, number>, from: Map<string, number>) {
+  for (const [k, v] of from.entries()) {
+    into.set(k, (into.get(k) || 0) + v);
+  }
+}
+
+function getTopLevelType(line: string): string | null {
+  const keyIdx = line.indexOf("\"type\"");
+  if (keyIdx === -1) return null;
+  // Avoid mistakenly matching nested `"type"` fields inside payload objects.
+  const payloadIdx = line.indexOf("\"payload\"");
+  if (payloadIdx !== -1 && keyIdx > payloadIdx) return null;
+
+  const colonIdx = line.indexOf(":", keyIdx + 6);
+  if (colonIdx === -1) return null;
+  let i = colonIdx + 1;
+  while (i < line.length && (line[i] === " " || line[i] === "\t")) i++;
+  if (line[i] !== "\"") return null;
+  const start = i + 1;
+  const end = line.indexOf("\"", start);
+  if (end === -1) return null;
+  return line.slice(start, end);
+}
+
+async function asyncPool<T, R>(
+  concurrency: number,
+  items: readonly T[],
+  worker: (item: T, index: number) => Promise<R>
+): Promise<R[]> {
+  const results = new Array<R>(items.length);
+  let nextIndex = 0;
+
+  const runners = new Array(Math.min(concurrency, items.length)).fill(null).map(async () => {
+    while (true) {
+      const index = nextIndex++;
+      if (index >= items.length) return;
+      results[index] = await worker(items[index], index);
+    }
+  });
+
+  await Promise.all(runners);
+  return results;
+}
+
 function formatDateKey(date: Date): string {
   const year = date.getFullYear();
   const month = String(date.getMonth() + 1).padStart(2, "0");