1. your memory is low. Ubuntu unity interface uses 3D acceleration by default. this could be what is slowing it all down. it's hard to say without more info.
2. you select the desktop type (e.g. Cinamon) on the same screen where you login to Ubuntu. note that cinamon also uses 3D acceleration.
3. the easiest way to check if the desktop is slowing it down is to isntall a different desktop. for example instead of reinstalling with Xubuntu all you need to do is find xubuntu-desktop in software center, install it and then on login choose xubuntu or XFCE desktop. if it is fast and responsive you can then remove default Ubuntu, Gnome & Unity desktop using a command line or finding the packages to be removed.
another thing you could to is install htop. then open terminal and run
this will show all processes and you can see what is hogging down the system. and if there is actually enough ram available.
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