If the game is slow or crashes try to put the quality of the game down. To do this you will find at the top right side of the game an optionsbutton in which selecting HD you will lower the quality to increase the speed of the game.
We also recommend:
Close the maximum number of windows/ tabs you have open especially when these are download programs.
Check your PC with an antivirus and antispyware to make sure that there are no malicious elements and that the antivirus is not blocking games. For more info click here.
Make sure you have installed the latest version of your browser: Google Chrome, Mozilla Firefox, Internet Explorer... and of your OS windows, OSX, etc.