Ahh flash games...
Try update your flash plugin to the latest version (untick 'Free Google Toolbar' unless you want that as well):
http://get.adobe.com/flashplayer/Clear your temporary internet files / cache.
Go to a clean arcade site (no other things that might cause issues like popups/adverts/scripting/etc) and pick a simple flash game to play. You can even go to my site if you wish - Aaron's Game Zone:
http://azzaboi.weebly.comOnce loaded, right click the flash object and select 'settings'. Ensure 'hardware acceleration' is ticked. Under the 'local storage' tab ensure it's set to 100kb or if it requires more increase this. Close and play the game.
Things like cursor animations in flash games can really slow it down depending on your mouse and drivers.
If the game is still lagging out, try again in full screen. Linking directly to the game itself. My site allows this as it helps reduce some computers lagging on some games like 'SAS: Zombie Assault 2' even when they are high specs.
Hope you solve your problem and have fun!