This is great news. I think Vim9Script is an excellent language. Just the fact that it will help speedup existing scripts is going to been a boom for Vim usage and plugin development.
I should start by saying my Vim config is reasonably minimal, but I'm highly skeptical of this making plugins noticeably faster. How much time are plugins spending executing Vimscript vs executing external tools?
I don't believe it'll speed up existing plugins though. If I remember correctly, vim9script is akin to an entire new programming language. Heck, even the syntax for comments has changed.
set number " This is a comment in vimscript
set number # This is a comment in vim9script
There's going to be a LOT of porting of existing plugins for this to all work.