Lighthouse measures your page under synthetic lab conditions. CrUX measures the last 28 days of real users from Chrome. When they disagree — lab green, field red — your testing methodology has a blind spot. This tool surfaces the gap for LCP, INP, CLS, FCP, TTFB.
Uses Google's public PageSpeed Insights API. Rate-limited without a key (5 requests / 24 hrs / IP). For heavier use, get a free API key at developers.google.com/speed/docs/insights/v5 and paste below.