Well, you can just install xfce4 or LXDE(lighter weight DE's) from the Software Centre and then login to whatever you want via the login screen. Onces you've installed them, just login out and pick one from the session option once you've pressed your user name.
Just remember it's still beta though. What exactly is slow? If your internet is slow, you can use OpenDNS and put the details in the network manager. https://store.opendns.com/setup/operatingsystem/ubuntu