Some (but not all) users have reported problems with 12.1x 64-bit Opera's stability and such, but not with Opera 32-bit versions... perhaps this is something similar. Those reported 64-bit "issues" are the reason I've remained with 32-bit Opera on my own 64-bit Win7 installation. Often times when something like this occurs, it's some kind of obscure driver-conflict issue that varies from system to system or software suite to software suite. In any case, you can assume that 12.1x will not be updated to intentionally fix this or any other performance bug-like behavior... from here on out, Presto Opera is only being security patched if needed.
Opera for Windows