Slow VIM help lookup

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.