I do not have any extensions, and I am, in fact, using the latest version of Flash. Also now, after a bit of testing, I found that the right click issue mainly happens while inside a flash window, such as a game or while on Youtube. I also tested and cannot use escape to close it (while in a flash window), so I guess it might be a problem with flash, although I doubt it. Another thing I realized afterwards was that it wasn't Opera 15 at all I had used. It was Opera 12. I also realized I have multiple versions of Opera installed, both 12, 15 and 22, though I don't think I ever used 15. Then I tried replicating the problem on the older version (12) and I did not have it there either, though I still have extreme lag on it.
As for leocg's suggestion, I disabled hardware acceleration, and it didn't fix it. However, it did increase my all-around fps by a bit (from 17-22 with still mouse and 30-34 while moving it to 23-26 and 40-42 fps).