perf: cache log timestamp formatters

src/logging/timestamps.ts

@@ -1,10 +1,21 @@
+const validTimeZoneCache = new Map<string, boolean>();
+const timestampFormatterCache = new Map<string, Intl.DateTimeFormat>();
+let hostTimeZone: string | undefined;
+
 export function isValidTimeZone(tz: string): boolean {
+  const cached = validTimeZoneCache.get(tz);
+  if (cached !== undefined) {
+    return cached;
+  }
+  let valid = false;
   try {
     new Intl.DateTimeFormat("en", { timeZone: tz }).format();
-    return true;
+    valid = true;
   } catch {
-    return false;
+    valid = false;
   }
+  validTimeZoneCache.set(tz, valid);
+  return valid;
 }
 
 type TimestampStyle = "short" | "medium" | "long";
@@ -18,7 +29,7 @@ function resolveEffectiveTimeZone(timeZone?: string): string {
   const explicit = timeZone ?? process.env.TZ;
   return explicit && isValidTimeZone(explicit)
     ? explicit
-    : Intl.DateTimeFormat().resolvedOptions().timeZone;
+    : (hostTimeZone ??= Intl.DateTimeFormat().resolvedOptions().timeZone);
 }
 
 function formatOffset(offsetRaw: string): string {
@@ -26,18 +37,25 @@ function formatOffset(offsetRaw: string): string {
 }
 
 function getTimestampParts(date: Date, timeZone?: string) {
-  const fmt = new Intl.DateTimeFormat("en", {
-    timeZone: resolveEffectiveTimeZone(timeZone),
-    year: "numeric",
-    month: "2-digit",
-    day: "2-digit",
-    hour: "2-digit",
-    minute: "2-digit",
-    second: "2-digit",
-    hour12: false,
-    fractionalSecondDigits: 3 as 1 | 2 | 3,
-    timeZoneName: "longOffset",
-  });
+  const effectiveTimeZone = resolveEffectiveTimeZone(timeZone);
+  let fmt = timestampFormatterCache.get(effectiveTimeZone);
+  if (!fmt) {
+    // Log timestamps are formatted on hot paths; Intl construction is much
+    // costlier than formatToParts, while timezone rules remain process-stable.
+    fmt = new Intl.DateTimeFormat("en", {
+      timeZone: effectiveTimeZone,
+      year: "numeric",
+      month: "2-digit",
+      day: "2-digit",
+      hour: "2-digit",
+      minute: "2-digit",
+      second: "2-digit",
+      hour12: false,
+      fractionalSecondDigits: 3 as 1 | 2 | 3,
+      timeZoneName: "longOffset",
+    });
+    timestampFormatterCache.set(effectiveTimeZone, fmt);
+  }
 
   const parts = Object.fromEntries(fmt.formatToParts(date).map((part) => [part.type, part.value]));
   return {