Currently the system runs 'makemandb -Q' in the background on every boot. This updates the apropos database (for 'man -k').
On an system with an existing man database this will stat(2) every manpage and update /var/db/man.db (an around 18MB sqlite database on a new amd64 system, maybe around 40MB on the same system with a reasonable selection of packages)
On a modern system rich in disk, memory and CPU this is unnoticeable. On a slower system (mac68k, vax, embedded 486), this renders the system effectively unusable for some time.
Given that makemandb is run nightly and weekly by cron anyway, it would probably be best for those systems to not have makemandb run on boot.