You'll have to ask Nik what they're doing during that time.
I had the exact reverse; took forever to load in CS5 but greased lightening with CS6.
When I installed CS6 I un-installed all my NIK products, checked they had really gone from CS5, then re-installed, the NIK installer found all bersions of CS5 and CS6 (32bit and 64 Bit) and all was OK. Assuming you have tried this approach and you still have slow loading of these plug-ins, I would reach out to NIK support.
But I can tell you NIK likes fast machines, on my old XP machine, Silver Efex Pro 2 took forever, and in use was a complete dog as well. When I upgraded to Win 7 loading was a way faster. I feel for you as I really did not want to spring for a new machine, but now glad I did.
May I ask which folder you put the Nik's softwear in? Adobe are telling me not to put it in the plug-in folder.