Market positioning
This is a comparison between two tools that mostly do different jobs, and pretending otherwise would be dishonest. WebPageTest is a performance laboratory. It loads a specific page in a real browser, from a real location, on a configurable connection, and reports exactly what happened: every request in a waterfall, every visual change in a filmstrip, every metric from first byte to Core Web Vitals. It has been the reference tool for web performance engineers for the better part of two decades and, under Catchpoint, remains free for core testing with paid API plans for automation and scale. SlapMyWeb is a website audit platform. Performance — including Core Web Vitals — is one of its eleven pillars, alongside crawlability, on-page SEO, content, links, mobile UX, structured data, international SEO, authority, SERP, and AI search readiness, plus accessibility and security modules. Where the two overlap is the performance pillar; where they diverge is everything else. The real question for most readers isn't which tool wins, but which job they're hiring for — and whether the honest answer is both.