Since non-XSLT has been removed, looks like part of C4::Search::searchResults() is only for staff interface. It whould be a better performance to skip it for OPAC. It concerns items loops @onloan_items_loop, @other_items_loop, @available_items_loop, starting around : https://git.koha-community.org/Koha-community/Koha/src/commit/7a7bee59021728a30c631e2ee51f9b7778217a9f/C4/Search.pm#L1833 We should try to add a 'unless $is_opac'. Maybe we could go further and move this to catalogue/search.pl, it would be more clear.
I haven't deep dived this one (yet) but (re)moving unnecessary code sounds good.