It should be fast enough for all modes. This isn't 1985. Computers in
general are *several orders* of magnitude faster than needed to do real
time syntax highlighting in general. Other editors don't seem to struggle.
Tree sitter is very fast. If regular editing is stuttering because of
fontification, we have bad data structures, algorithms, or architectures
--- that is, bugs. And we shouldn't add user options to paper over bugs.
That's ridiculous. I can't believe we really want to propose a "please make
syntax highlighting wrong" user option.
I'm all for keeping context in mind, and I think that part of that is Eli's unusual circumstances: running unoptimised builds with extra checking enabled. I don't know what his particular hardware is like, but my laptop is a medium-spec i5 from ~4 generations back running debian inside a lightweight VM, and I can both scroll from top to bottom of src/xdisp.c and open the file and immediately Esc-> to the end without (being aware of?) font-lock falling behind.
Are other people having much worse experiences than this? Is there some other situation where emacs developers are frequently seeing problems? I don't do anything with C++ anymore, and I haven't bothered setting up LSP here.
Thanks
~Chad