Jetbrains have a code indexer which is what slows things down at launch. After that, it’s fast. Also I have those things opened for days. Emacs’ uptime is 7 days now (I restarted it to revert a config changes, the mbp itself is up 35 days).
VSCode with clangd is supersonic compared to CLion with clangd-indexer.
Indexer only actually runs if you update the IDE, otherwise it does incremental update on the changed files, which even if you switched to a branch with many changes is usually instant (or too short to realise it's happening). They optimised indexing a lot over the years.