With LR 4.1 my IQ180 files took about 10 seconds to display in the Develop module - the Loading... message seemed to take forever. Zooming to 100% was pretty quick once it was loaded ... sharp right away. (12core 3ghz MacPro with PCI SSD drive with OS and apps and 4 drive raid for images storage, 64 gigs of ram).
It appears LR 4.2 changes the process so it is substantially faster ... if I'm at Fit or Fill and move to another image, it only takes about 4 seconds for that image to appear and be sharp. If i immediately zoom to 1:1, it will take 4 or 5 more seconds. If however I wait for 5 or so seconds, then zoom to 1:1 it's almost instant. So it appears to be building a 1:1 preview in the background and doesn't wait to display the screen resolution one for that 1:1 to finish. (certainly speculation on my part , I'm sure the engineers have a much more elequont way to describe what they did ... whatever it was TY TY TY !)
The local adjustment brush also seems snappier, and the develop module sliders seem to update much more robustly ...
for me, LR 4.2 is much snappier. d800 files and NEX 7 files only take a second or so.