Good news, the slowness can be easily resolved by compiling the query pattern in csharp-mode-font-lock-settings-1 (this is a recent addition to treesit).

Yuan

Thanks for the reply and thanks for looking into this.

I can confirm that by compiling the query like you suggested, and replacing linum-mode with nlinum-mode, I'm not experiencing any performance issues any more!

To avoid issues like this... Should perhaps the function treesit-query-capture (in treesit.c) emit a warning/message when encountering non-compiled queries?

That way writing more performant major-modes would be more self-explanatory, resulting in a better, faster Emacs for everyone.