docs: add comment about possible performance boost

Author: ravibrock

lua/spellwarn/diagnostics.lua

@@ -11,6 +11,7 @@ local function get_bufs_loaded()
     return bufs_loaded
 end
 
+-- PERF: try wrapping this with a function to make it run asynchronously?
 function M.update_diagnostics(opts, bufnr)
     if opts.max_file_size and vim.api.nvim_buf_line_count(bufnr) > opts.max_file_size then return end
     local ft = vim.fn.getbufvar(bufnr, "&filetype")