Poor Safari Scrolling Performance

When I visit Ars Technica or The Verge, I get heavy lag on my MacBook Pro. I'm running a 2011, 2.3GHz Intel i7-2820QM model with 8GB of RAM on Mountain Lion (OS X 10.8). Scrolling in Safari 6 is normally locked to 60FPS, but the framerate drops to what feels like an unsteady 20-30.

These sites work perfectly fine on IE9 or Android 4.1's browser. It's only occurring on Mac OS X 10.8 with Safari 6.

I'm running the AdBlock and Facebook Cleaner extensions. I also have the latest version of Adobe Flash Player installed. This problem occurs even when no other tabs/programs are open.

Also, when scrolling quickly through these websites, Activity Monitor shows up to 79% CPU usage for the Safari Web Content process.

This problem occurs on both the Intel HD 3000 and the AMD 6750M, although the 6750M seems to reduce the lag considerably.

Does anyone have any idea what is causing this problem? Some responses to a similar post on MacRumors shows that other users are not able to replicate this irritating problem.

Thanks for the help.