With this .vimrc:nnoremap [ :lp^Mnnoremap ] :lne^Mfiletype plugin onvim 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.
`filetype plugin on` and square brackets
key bindings;vimrc;filetype
Thanks to @romainl, it turns out the delay is caused by ftplugin defining mappings starting with [.