When i first go tLR4 i had to upgrade my PC for it to be usable. All other programes seemed ok.
when i got the enw hardware it seems much better, but i still find it a bit sluggish going between different modules, and evne switching between diffeenrt image in the Develope module.
Its usable though and im used to it. The funny thing is my work Pc, that has a lower spec seems quicker to skip though images. Most of the slowdowns do seem to be to do with preview rendering. i read about keeping NR and the clarity slider off until the last min but i never found them to be an issue. This PC at work (3 actually and all seem slightly faster with LR, slower with everything else, as to be expected) only have 4GB ram (W7 Pro 64), as did mine at home. I upgraded to 8GB but that made no difference at all.
I loaded all this months raw files in to LR5 so it more or less mirrored LR4 and there is definitely a difference. I wouldn't say its huge, or if its because ive only got about 200 images in it so far, but it seems faster to get around.
There are something that dont work properly yet, but so far ive just found these to be minor niggles.