The default desktop environments on all three of our recommended distributions (RHEL7 & SLES12: gnome-shell, Ubuntu 16.04LTS: Unity) all have performance issues making them less than ideal. We recommend these distributions because they are good choices for long-term deployments, and they come with support offerings. The problem is that neither of the distributions has standard desktop environments them are suitable for ThinLinc at this point.
If we were to recommend alternative desktop environments, my opinion is that we should strongly suggest alternatives that comes packaged from the OS vendor itself rather than third-party packagers, to keep recommending systems/setups that are supported by a vendor. * Ubuntu LTS has plenty of alternatives. * RHEL7 comes with packages for KDE. * SLES12 has no known alternative desktop environments.
*** Bug 7844 has been marked as a duplicate of this bug. ***
Note that this might not be a universally bad experience with modern systems. Using GNOME on Ubuntu 20.04 on our new servers is actually quite pleasant. (it's still very inefficient compared to e.g. Xfce, but it is clearly possible to throw enough hardware at the problem to get a decent frame rate)
