LR 4.3 works fine on my Win7 64 bits, 12 GB Ram machine, no decrease at all compared to LR3. If you have the right configuration, LR4 will do the job with ease. Adding more Ram (at least 10 GB) is the first thing to do if LR lags.
The LR4 basic panel in the Develop module (Process 2012) has been reprogrammed: the new Shadow and Highlight sliders are awesone, the exposure slider works now on global midtones. The Clarity slider was also reprogrammed and it's does a better job (you can push it to 100 without creating artifacts such as halos on some pictures).
And quite important too: the Graduated Filter and the Adjustment Brush are much more powerful since you can adjust 12 parameters (6 parameters only in LR3)
And the Tone curve allows individual adjustment for each color channel, a big improvment.
So LR4 is worth the upgrade. And don't forget this: if you have LR3 develop presets you like, you can use them in LR4 but you will have to switch to the old Process 2010. And LR4 develop presets will not do the right job in LR3 since the sliders in the basic panel are not the same.
Another thing about presets: if you work with PS CS6 and LR4, you can export your LR4 develop presets in ACR, since ACR is exactly the same engine as the LR4 Develop module .