Cognitive slowing is really hard to deal with. For me, depression is what slows me down; thankfully, meds don't affect me cognitively too much.

If you are certain that your meds are causing this, then I suggest that you try other combinations. There are so many meds out there that can treat bipolar; I'm certain that some of them won't have that terrible side effect! If worst comes to worst, and your bp is mild, you might want to consider going off meds. If you can't function due to side effects, then something needs to be done.