If you don't use a huge toolkit, a modern browser is massively faster – I last did benchmarks like 8 years ago and even then it was usually an integer multiple more memory or CPU for something done in Flash versus the browser and that was before accounting for the browser having better quality text rendering, color management, alpha effects, antialiasing, etc. Had Adobe not just stopped investing after they hit 90% marketshare that might have been different but they were sitting still for too long.

The problem is that having made things like JS, the DOM, CSS, etc. so efficient just increased the threshold before performance forces developers to notice inefficiencies but that's a pretty portable problem, too — we had to force Flash developers to test on older systems with slower connections the same way.