“An increase in execution speed of 10 to 100 times can be expected.”
Vim script parsed every line every time it ran. Vim9 script compiles it, at the cost of full backward compatibility. That is a hard trade for an editor whose whole appeal is that nothing breaks. More than 5,000 patches since 8.2 had already shipped to anyone building from source, so most users are getting the release they’ve been running for months.