So I grabbed my computer and placed side-by-side I compared my liquify tool in CS5 on my i7 MacBook Pro 6.2 with 8GB RAM. Surprisingly, the tool worked much better...
The secondary display.
When testing against my friend's MBP 5.2, I unplugged everything and brought the computer into the kitchen. When I replugged the MBP to the Eizo (via Mini-DisplayPort>DisplayPort), the liquify tool lagged again.
I'm not sure but potentially the GPU is being underused and I have heard that the GPU in the new MBPs are not supported by Photoshop CS5. I will do more research and report back. At least I've narrowed down the issue.