You're using the open source radeon driver .. the proprietary fglrx (catalyst) would probably be better....
(it would also likely provide hardware decoding)
I'm not familiar with Zorin, but is there something in the menu's called "Addition Drivers", or "Software & Updates" ?
If you can't find anything in the menus, try opening a terminal and running:
If a dialog opens, select the "Additional Drivers" tab.
Are there any drivers listed there for activation ? .. if so what ?