You're still comparing Flash on twenty year old hardware to HTML+CSS on modern hardware.
Of course modern computers are orders of magnitude more powerful! But Flash was definitely generally worse compared on the same hardware and network stack compared to vanilla (non-plugin based) web tech.
I feel like people are talking past each other a bit here. FlashScript was never very fast, and rendering a document as a giant collection of bezier curves was not fast, but the people doing animations with it were getting the equivalent of modern day CSS3 animations + SVG, and it ran nicely on hardware two orders of magnitude slower than what we need for CSS3+SVG