Measuring CORS Preflight Latency in Production

The cost of a CORS preflight is real but easy to overlook: on a cache miss, every non-simple cross-origin request pays an extra OPTIONS round trip before the actual request is even sent. In development that latency hides behind a fast local network; in production, across real networks and CDNs, it becomes a measurable slice of API latency. Measuring it is what turns preflight optimization from guesswork into a number you can track.

This page is part of Preflight Performance Analysis, which explains the mechanics of preflight cost and how Access-Control-Max-Age amortizes it.

The Symptom This Resolves

You suspect preflight is inflating your cross-origin API latency, but you have no number. Perhaps p95 latency for a POST /orders from the browser is far higher than the same call measured server-side, and the gap looks like one extra round trip:

Server-side p95 for POST /orders:      42 ms
Browser-observed p95 for POST /orders: 118 ms   ← where does the extra ~70 ms come from?

Root Cause

Per the WHATWG Fetch Standard, a non-simple request triggers a preflight whose result is cached per (origin, URL, method, header set) for the duration of Access-Control-Max-Age. On a cache miss, the browser serializes the preflight before the actual request, so the user-perceived latency is preflight RTT + actual RTT. The browser-versus-server gap above is that preflight RTT. If your cache hit ratio is low — because Access-Control-Max-Age is short or a per-request header keeps changing the cache key — most requests pay it.

Prerequisite State

• Cross-origin API calls are already working (this page measures cost, it does not fix broken CORS).
• You can deploy a small client-side script or already run a RUM/telemetry pipeline.
• You have access to server or CDN access logs that record the request method.

Step-by-Step

Step 1 — Instrument the client with the Resource Timing API

A PerformanceObserver captures timing for every resource, including cross-origin API calls. The preflight itself is not a separate entry, but a large gap before the response phase on the actual request signals a cache miss:

const API_HOST = 'api.example.com';

const observer = new PerformanceObserver((list) => {
  for (const entry of list.getEntriesByType('resource')) {
    if (!entry.name.includes(API_HOST)) continue;

    // Time spent before bytes start flowing back for the actual request.
    // On a preflight cache miss this includes the OPTIONS round trip.
    const preConnect = entry.requestStart - entry.fetchStart;
    const waiting = entry.responseStart - entry.requestStart;

    navigator.sendBeacon('/rum/cors-timing', JSON.stringify({
      url: entry.name,
      fetchToRequest: Math.round(preConnect),
      ttfb: Math.round(waiting),
      duration: Math.round(entry.duration),
    }));
  }
});

observer.observe({ type: 'resource', buffered: true });

Step 2 — Log OPTIONS requests separately on the server

To isolate the preflight precisely, record OPTIONS requests with their timing. In Nginx, add the method and request time to the log format:

log_format cors '$time_iso8601 $request_method $uri '
                'status=$status rt=$request_time origin=$http_origin';

server {
  location /api/ {
    access_log /var/log/nginx/cors.log cors;
    # ... CORS handling ...
  }
}

Step 3 — Compute the preflight cache hit ratio

Compare the count of OPTIONS requests to the count of actual requests to the same paths. A low OPTIONS-to-actual ratio means the browser cache is doing its job:

# Over the last log window, per method
awk '{print $2}' /var/log/nginx/cors.log | sort | uniq -c
# e.g.  1200 OPTIONS   /   18400 POST   → ratio ≈ 1:15, healthy
#       9100 OPTIONS   /    9800 POST   → ratio ≈ 1:1, cache barely hit

A ratio near 1:1 points at a short Access-Control-Max-Age or an unstable header set — tune it per How to Set Access-Control-Max-Age Effectively.

Step 4 — Alert on regressions

Feed the beacon data and the log ratio into your metrics pipeline and alert when either the OPTIONS p95 latency or the OPTIONS-to-actual ratio rises — a common regression when a client library starts adding a new custom header on every request, breaking the cache key described in Reducing Preflight Frequency with Header Caching.

Verification

# Confirm OPTIONS requests are being logged with timing
grep OPTIONS /var/log/nginx/cors.log | tail -5

• The RUM beacon fires for cross-origin API entries (check the network log for /rum/cors-timing).
• Server logs record OPTIONS separately with request_time.
• The OPTIONS-to-actual ratio is well below 1:1 (cache is being hit).
• A dashboard tracks OPTIONS p95 latency and the ratio over time.

Security Boundary Note

When you log the Origin header for analysis, treat it as untrusted input — it is attacker-controllable and must never be used to make an allow decision, only to observe one. Do not build a dynamic allowlist that trusts whatever origins appear in the logs; keep validation explicit as described in Dynamic Origin Validation Patterns. Also avoid logging credentials — never record Authorization or Cookie values alongside the timing data.

Common Mistakes

FAQ