Add this line CLUTTER_PAINT=disable-clipped-redraws:disable-culling to the file /etc/environment
Restart your session and you should now be able to was Flash videos in full screen tear and lag free.
Bug is registered here:
https://bugzilla.gnome.org/show_bug.cgi?id=657071
https://bugzilla.redhat.com/show_bug.cgi?id=720605
