[imaltont]

In my experience it depends a bit on the LSP server itself (as well as it just being horribly slow in Emacs 27 and older). Omnisharp is for instance way slower than clangd and rust-analyzer. Also helps to make sure all the various mingw dependencies are there if you use Windows (json and native compile in particular). If you used LSP-mode it would be worth checking out if you can disable some of the information it renders to improve performance in general.

Other than that, Company can be a bit slow compared to Corfu, which is a bit more lightweight completion framework, built more around the emacs internals.