This doesn't sound like anything to worry about. And it likely has nothing to do with permissions.

You could run the monitor program to see where else your system might be stressing out. For instance are you swapping out memory because you don't have enough.

Some sites have very complicated layouts, not to mention a ton of images to download and scripts. That could be causing the browser to perform more work.

Try different browsers on the same sites to see how they perform.