It is a boot-time tracing facility. See https://cgit.freebsd.org/src/commit/?id=46dd801acb2318c8464b185893bb111f09b71138 (there are probably more relevant commits in the src tree log).
Yep, no documentation here: % grep -r -i "TSLOG" /usr/doc/documentation/ % Elsewhere ========= <https://wiki.freebsd.org/BootTime> <https://github.com/cperciva/freebsd-boot-profiling> Comments from Colin Percival include <https://old.reddit.com/r/freebsd/comments/rya5pz/-/hsxy91c/> re: configuration and <https://old.reddit.com/r/freebsd/comments/s67ymv/-/ht21si7/> re: modernity of the boot loader.
https://www.freebsd.org/cgi/man.cgi?query=tslog&apropos=0&sektion=0&manpath=FreeBSD+14.0-current&arch=default&format=html