It appears that Neomutt can be quite slow when filtering/searching emails using body keyword or patterns and the “limit” (l) command. It seems that Neomutt needs to open all emails beforehand, which makes sense but slow down any search operation. Now, as suggested in #3273, we can speed up things a lot by disabling the auto_view text/html feature. My benchmarks confirm those of the poster, with around 13K messages in my Archives. I don’t know how to disable auto_view temporarily, but since I can use mu-find to search my email database, with almost the same syntax, I do’nt have to worry much: results are almost instantaneous!
macro index \Cf "<shell-escape>mutt-mu " "mu find"
macro index gf "<change-folder-readonly>~/.mu/results<enter>" "mu find results"
Here, mutt-mu is a short shell script for mu find --clearlinks --format=links --linksdir=~/.mu/results "$1".