I agree on security and bugs, but bugs can be fixed. It just shows neglect by Adobe, which was, I think, the real problem. I think that if Adobe seriously wanted to, it could have been a web standard.
Those did sometimes run really great, but most implementations were indeed very slow.
I remember vividly because it was part of my job back then to help with web performance and when we measured page speed and user interface responsiveness flash was almost always the worst.
You remembering a few optimised instances does not change the reality that Flash was bad.
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