Regex-based syntax highlighting is usually the culprit in these cases. There’s probably a new RegEx inside c++ mode that burns too many cpu cycles.
You might want to try out the treesitter mode „c++—ts-mode“ that does the syntax highlighting based on a real static parser written in C: https://medium.com/really-learn-programming/setting-up-tree-...
Disclaimer: I’m not a C++ dev so I have never tried that.
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/sigh
I am still not a competent C++ programmer but this is how your basic tree sitter C++ mode is supposed too look with "C-h m":
Minor modes enabled in this buffer: Auto-Save Flycheck Flyspell
Font-Lock Yas
The major mode is C++// mode defined in c-ts-mode.el:
Major mode for editing C++, powered by tree-sitter.
This mode is independent from the classic cc-mode.el based
‘c++-mode’, so configuration variables of that mode, like
‘c-basic-offset’, don’t affect this mode.
To use tree-sitter C/C++ modes by default, evaluate
(add-to-list 'major-mode-remap-alist '(c-mode . c-ts-mode))
(add-to-list 'major-mode-remap-alist '(c++-mode . c++-ts-mode))
(add-to-list 'major-mode-remap-alist
'(c-or-c++-mode . c-or-c++-ts-mode))