Magic bullet effects applied to clips in premiere will slow down and be, at best, choppy on much faster computers than yours.
As mentioned, if you render it should solve the problem, but can be time-consuming especially if you then go on to edit with the rendered clips.
The new Magic Bullet Editors 2.0 is supposed to largely solve this problem, given that you have a decent GPU graphics card.
If you can't stretch to MB2.0 + graphics card, then i'm afraid you're stuck with it.
The best you can do is make sure you do ALL you editing first and then apply any MB effects after everything is in place - then you only need render once.