6 years old is not that old. My old laptop was a budget laptop from 2007 and
it worked with MATE, Cinnamon, GNOME, etc just fine. Pretty much any x86
computer from 2012 should easily be able to handle any modern desktop
environment, even KDE. In fact the newest hardware that can work with a 100%
libre software stack (that is to say, the Libreboot-supported computers) are
older than that.
Loading speed is influenced almost entirely by how fast your hard drive is.
How many seconds are you talking about with regard to loading speed? If it's
more than a few seconds or so I would seriously consider replacing that hard
drive since such slow operation could be an early sign that it's developing
too many bad sectors. Using a lightweight DE that loads less stuff is only a
band-aid solution and isn't going to protect you from the inevitable data
loss.
On the other hand, if it's only a few seconds that's bothering you, that's
because you're using a hard drive, not because of the age of your computer.
You can get faster read/write speeds with an SSD.
Also, Xfce is available on Trisquel; just install the "xfce4" meta-package.
