`filetype plugin on` and square brackets

Question

With this .vimrc:

nnoremap [ :lp^M
nnoremap ] :lne^M
filetype plugin on

vim becomes sluggish when switching between "errors" of location list with square brackets. As opposed to using lprevious/lnext directly. The same goes for mapping brackets to cprevious/cnext.

Everything is okay without enabling filetype plugins (just filetype on). Or if filetype plugin on is executed after vim has started. Or if vim is started as follows: vim -u ~/.vimrc.

Answer 1

Thanks to @romainl, it turns out the delay is caused by ftplugin defining mappings starting with [.