> Hi, I've seen a couple of ideas in the internet on how to spped his up. You
> could use the buffer-change hooks and defadvice on undo and would be much
> less than on every keypress, right? I think this could be a good compromise
Fortunately, it was much easier to implement this by adding
just these 3 lines to tab-line-format:
+ (and (memq 'tab-line-tab-face-modified
+ tab-line-tab-face-functions)
+ (buffer-file-name) (buffer-modified-p))))
So now this is fixed in the emacs-28 branch.