In one of the recent smaller patches, since 3.3, they announced a change where warcraft will use as many cores as your processor has, instead of only 1 or 2 cores.

That's the only change I can think of, which relates to cpu cores.

There was something you could change in the Config.WTF file to revert to the previous behavior. I'd check the patch notes, for the most recent patches and see if that fixes your issue.