Browsers are slow because objective metrics of quality (e.g. performance, usability) are irrelevant to the common user, despite what individuals may claim. People are very susceptible to branding and growing trends, and it's a hard pill to swallow.

Back in the day when I was writing on the forums "guys, forget Firefox and use Opera, it's fast it's usable you have everything you might want in a browser with undeniable proof", the reply would eventually be "yes but Firefox has _this_ webdev extension and I can't do without it". So Firefox would get installed in their families' and friends' PCs, because that's what the web developer used. Also, the "safer faster better" marketing campaign, although that slogan would have been much more appropriate to Opera than Firefox. Fast forward a few years. Google Chrome comes out and everyone stops caring about those damn extensions; suddenly performance becomes a huge decision factor (nevermind Chrome was a resource hog compared to Opera, nevermind the horrible lack of features, nevermind the bad usability). I could go on, but you get the drift.

There's no incentive for large market share browsers to significantly improve because the (mediocrity+marketing=success) recipe is proven to work, over and over. What would it take for Chrome to lose market share? A new hot hip popular company with lots of money thrown in advertising, like (extremely unlikely example) Tesla, with a new cute interface. Because people like new and shiny trinkets.