There are several things that might make the games on this website run slow.

1. Your Connection
2. Too Many People On Server
3. Slow Server
4. Your Computer
5. Your Internet Browser 
6. Your Internet is too slow to run script- Fix:This might sound crazy, but play the game in fullscreen mode
   

Here are some links that might help.
Game running slow?-Try reducing the quality settings in the game (if the game gives this option.)  Some games require a very fast computer to play well at high quality.
For Firefox-http://board.flashkit.com/board/showthread.php?t=605490

If games don't run at all try downloading the latest adobe flash player at- http://get.adobe.com/flashplayer/?promoid=BUIGP
or
Try clearing out your browser cache and then going to the game you’re having trouble with and clicking on the refresh  button in your browser.