The default, and by far the most common, output from Flash had significantly slower click-to-response and for network latency and for rendering than HTML+CSS is today.
You remembering a few optimised instances does not change the reality that Flash was bad.
I am not and have never compared them in the way you say I did. You literally wrote ”Or that it was worse than the performance of doing the same thing in modern HTML+CSS now.” so I had to somehow repsond to that strange claim.
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.
Maybe at rendering menus and documents, but flash had graphic routines written in optimized assembly that simply weren't possible with JavaScript on that era of hardware.
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
You're still comparing Flash on twenty year old hardware to HTML+CSS on modern hardware.