I've started to notice that Opera is seeming to struggle a bit, it takes ages to load (I could probably load Netscape AND IE in the time it takes!) and switching between pages in it and other tasks are really sluggish.
I'm thinking of reinstalling it but wondering if anyone else is having a similar problem, i'm frankly suprised that Opera would have the microsoftian behaviour of slowing down after a while. ;)
The biggest thing is cut down on the length of time that links are marked visited:
prefs -> documents -> link presentation -> visited links marked
Set that to about 7 days and exit/restart opera. That info is stored in "vlink.dat" in your Opera directory. If it gets to large, it can take a long time for Opera to check links between the cache and visited links - slows down general surfing and startup time.