installed 2.5.3 today and when we pulled the 1st boss in raid i had massive fps loss to average now 12... rebooted no diff, tried disabling a few addons no diff.. when i disable plexus it was fine (tho a nightmare to heal:)
i rolled back to the version 2.5.2 and everything is ok again.

is 2.5.4 any better?

I will be able to test on tuesday as that's when i next raid (wasn't noticable outside raid)