What is screen resolution?
If you turn off some panels (especially Lens Correction and Details) and Highlights, Shadows and Clarity, delele any local corrections, does is still lag?
I have and older dual-core CPU (a Clarckdale Core i3 550), quite a small 1280x1024 screen and Win7 x64. My basic sliders move quite smoothly in Develop in Fit mode with just the right panel showing. I'd say LR4 does about 10fps on 10 Mpx files and ~4 fps for 36 Mpx.
Subjectively, Lens Corrections decrease performance by some 20%, all detail sliders take away another 20%. Grain might eat some performance too. Local correction can range from moderate to medium hit, depending on amount and controls used. And all these feel about the same as LR3.
But the biggest hit is taken when using new 2012 controls: Highlights, Shadows and Clarity. If I use any of these, I can see perhaps a 5-10 fold drop in performance. LR3's Recovery, Fill and Clarity were not near as CPU-intensice as that.
I think you might be experiencing a bug. I create new catalogs for all my shoots, but a normal shoot with approx. 1000 images runs quite smoothly on my field laptop, and thats a Core 2 Duo 2.5Ghz with 4GB's of RAM. Almost an ancient machine, but great for fieldwork...
Reinstall it maybe or reset the preferences?