I want to believe this / hate on Apple as another iPhone X user with very slow web performance.

I would wager it’s not Safari mobile though just your standard website got way more resource hungry due to SPA proliferation etc.

TL;DR hard to verify your claim as Safari is the only allowed browser engine on iOS.

If somebody had a jailbroken/dev phone that would eg allow compile and install of Firefox then we could actually know.. I’m very skeptical it’s down to the browser though. Not sure if typical benchmarks are telling as hardware always different. Again “badly optimized” websites and 6+ years old hardware; albeit should be fast enough…