Three tools, three philosophies, one website — different numbers. Here is what each measures and the workflow that finds real problems.
PageSpeed Insights (Google) - Measures: Core Web Vitals (lab) + real-user field data from Chrome - Trust for: the official ranking-adjacent numbers, mobile reality, before/after comparisons - Weakness: field data needs traffic volume; new sites show 'not enough data'
GTmetrix - Measures: full waterfall — every file, its size and load order - Trust for: finding THE file slowing your page (that 4MB hero image, the rogue script) - Weakness: test location defaults abroad — set a nearby server for Pakistani reality
WebPageTest - Measures: filmstrip view — what renders, in what order, second by second - Trust for: understanding what visitors SEE at what second; 3G/4G simulation - Weakness: interface density — overkill for weekly checks
The workflow that works (monthly, 15 minutes) 1. PageSpeed Insights mobile: the score trend (is it improving?) 2. GTmetrix waterfall: the top 3 heaviest files — fix or compress 3. WebPageTest filmstrip: quarterly, for the 'what does the visitor see' reality check
The number to trust above all Your own analytics: bounce rate on mobile pages. If speed fixes move bounce down and pages/visit up, the numbers were real. If business metrics did not move, the speed was never the bottleneck — look at content and offer instead.
Tools inform; business metrics decide. Run the trio quarterly on a Launch-tier site — everything you need is free.