Lately (I think) VIM helps load very slowly during the first lookup. But it only happens at the first lookup, I just ignored it since I was usually in the middle of doing something else.
Anyway, I had a look at my
vimrc last night and tracked down the cause.
autocmd BufRead,BufNewFile README,TODO,*.md,*.txt setlocal filetype=markdown
Remove this and the lag was gone. Hmmm.