I have been using LR4 and now 4.1 since beta and subsequently the first release and now 4.1RC. I too have been irritated by the lack of responsivness compared to LR3.6 but in 4.1 it does seem to have improved. BUT the problem I am seeing is that the preview image in develop module chnages to a lower res and muddier looking version as I drag most of the various adjustment sliders, once the slider is realeased the image refreshed to a better resolution version. This happens in all panels from Basic to Detail and even Effects. It is almost as if Adobe are increasing the apparent speed by applying the adjustment to a low res embedded file first and then to the full RAW data in the background. On detailed images it is more irritating that the slow develop module in LR4.0 as it makes adjustments impossible to preview until the slider has been deslected. The same occurs if you select the slider setting digits and advance using the cursors. Very frustrating indeed. I will be going back to LR4.0 and fingers crossed that this is resolved when LR4.1 is reeased in full.
My PC is fairly good, Win 7 64bit with Quad Intel 9550 cpu (2.83Ghz) with 8Gb ram, GTX-260. I have the same problem at work with a dual cpu (8 cores total) Zeon 5365 (3.0Ghz) and 12Gb Ram & Quadro FX4600. The problem does not seem to manifest in the second display preview but this is laggy and obviously not any use for setting local adjustments.
Anyone else had a similar issue to this? I cant find any reference to it on the web or these forums.