The GUIs work fast and beautifully in studio, but not in play mode. I understand that it would be laggier in servers, but is there anyway to reduce the lag on the Tweening effects to make them more efficient?
Make sure that your script is a local script. This will put the tweening processing on the user's computer, not the server.
See this wiki page for more info: http://wiki.roblox.com/index.php/Tweening