Nice has no noticeable effect on FreeBSD. As far as I understand from past mailing list discussions about scheduling, the schedulers don't give it a lot of credit.
What you can do is impose limits in your login.conf. But maybe that should be done in a jail. I do not know the consequences of imposing such limits on the root user.
: binary package maintenance, library integrity checking, ...
: [man=8]amd[/man] based automounting without HAL
: My posts represent my perception. Errors and incompleteness are to be expected, I deny any responsibility to know everything.