According to this doc, _fini
is a function that used to be included in libraries, and it would be called whenever the library was unloaded.
It can also be a symptom of lots of objects being created and destroyed (under the covers where you can't see it).
gprof
often points to functions when the reason for the slowness is elsewhere.
That's just one of its many problems.
If you're trying to eliminate waste cycles from your program, here's what a lot of people do, and it works.