Yes its 4-6%, and you are right its not much.
Opera has never been very demanding on my CPU usage, which is one of the reasons why I use it.
Though it seemed to have a memory issue on my old computer where it would use almost half the available RAM, though this don't seem to be a problem anymore.
The CPU usage does happen in a cycle as you say, it goes up to the 04-06 range, and then drops in increments to 01-00, and this happens in a regular cycle of a few seconds.
The Antivirus I use is Panda.
It has a process surveillance system that can track the URL access of each process, though I had to turn that off earlier as a bug in Panda it was causing bluescreens when that option was turned on.
I installed an update for it, but have yet to turn that option on.
A second option in the same category is simply a process surveillance, though I'm not sure what it does, its the first time I use Panda so have yet to learn it.
Other than that there is a safe surfing category that's turned on, that seems to be about what sites are trusted and not.
But I would have thought that if Panda was the culprit, deactivating it would have had an effect on the problem, something it did not have