He probably would have better results with Xubuntu or Lubuntu, but the lack of that Long term Support is probably why he just wants a different DE. (Which is understandable, most do not like changing their OS often, or even upgrading often, because it can be buggy.)
It should be over all faster, boot times will still be the same from my understanding. But when in use, it can effect performance. Because you only use the session you choose. If I use my LXDE session, that is what it is doing, if I use XFCE, that is what it is is using. You can install any DE basically you want, but it is pretty hard to remove them from my understanding, and it can bloat it and give you dupes of packages. So go ahead and try another DE, and if you are good at working with the command line, use a minimal install and work your way up to make an Ubuntu for you, light weight, with LTS.