Find slow Neovim startup fast with --startuptime self-time sort

To see why Neovim feels slow to start, run nvim --headless --startuptime st.log +qa and sort lines by the second column (self+sourced ms) to get the heaviest require('...') calls. In a Nix-managed config with no lazy loader, eagerly required test frameworks (neotest + language adapters ~90 ms) and AI/UI plugin files dominated a 0.6 s start. Time real file opens too (time nvim --headless <file> +qa), since ftplugins/LSP/treesitter add ~0.3-0.4 s. Ask whether 'slow' means startup or typing lag before fixing; they have different causes.