You'll have to file a bug report, because none of the built-in LSP stuff should cause any performance degradation.
> syntax highlighting just stops working randomly
This has nothing to do with LSP.
> files get spewn all over my repositories that I have to .git/info/exclude
Neovim does not automatically create files in your repositories.
That's a bold claim. Migrating from NCM + clangd integration is absolutely slower.
> This has nothing to do with LSP.
Sorry, you're right. Tree-sitter-based syntax highlighting is buggy and slow. My point still stands.
> Neovim does not automatically create files in your repositories.
No, you're right. Clangd does. I was commenting more on my experience, less Neovim's LSP behavior specifically.
You'll have to define slower (time to diagnostics, time to completion, are you really talking about the performance of nvim-cmp instead, etc.) preferably in a bug report with benchmarks. I've done several rounds of performance optimization since 0.5, and I haven't heard any complaints since 0.6 when I swapped out our json decoder.