There are STB's that aren't slow and laggy? Seems like all I've used are slow and laggy... cable or satellite (Charter, Cox, DirectTV, even a couple different models of Sky+ boxes).
Yet another reason why I don't subscribe to cable or satellite... my OTA DTV receiver (I have an old SD TV) is nice and snappy (although nowhere near as full of useless features as some cable/satellite STBs).
On the subject of MediaRoom: IIRC, AT&T's Uverse service uses MediaRoom on their STBs... don't have any experience with that service, though; it's not available in my area yet.
Have seen a demo of Mediaroom-based service, and from that can say that it is not too laggy (and compared to DVB boxes, its super-instant). But things looks much better in demonstrations than they really are. So, can't say for sure how it behaves on regular