chore(core): add location startup diagnostics

packages/core/src/observability/event-loop-delay.ts

@@ -0,0 +1,27 @@
+const ResolutionMs = 20
+
+export function makeEventLoopDelayMonitor() {
+  const state = {
+    expectedAt: performance.now() + ResolutionMs,
+    maxMs: 0,
+    stoppedAt: undefined as number | undefined,
+  }
+  const timer = setInterval(() => {
+    const now = performance.now()
+    state.maxMs = Math.max(state.maxMs, now - state.expectedAt)
+    state.expectedAt = now + ResolutionMs
+  }, ResolutionMs)
+  timer.unref()
+
+  const maxMs = () => Math.max(0, state.maxMs, (state.stoppedAt ?? performance.now()) - state.expectedAt)
+  return {
+    maxMs,
+    stop: () => {
+      if (state.stoppedAt === undefined) {
+        state.stoppedAt = performance.now()
+        clearInterval(timer)
+      }
+      return maxMs()
+    },
+  }
+}