Initially I opened Lags when navigating vc-root-diff buffer and the cause of slowness was Emacs opening referenced files by diff.
Back then I used Emacs 27.1. Now I'm on Emacs 28.2 and previous solution:
(setq diff-font-lock-syntax 'nil)
- I experience delay of Emacs opening files in the background
- later I end with lots of buffers with files I personally never opened but Emacs opened them for me, so I spend minutes closing tons of files, they pollute history of recent files (I don't care about robotic opened file, I need only human origin).
UPDATE 2023-04-10 Bug report: Emacs keeps opening related file from vc-diff buffer