Is there some way of configuring "No Effects" in the backend of GNOME Shell to get less memory/CPU overhead from graphical applications?
I don't dare to turn on Metacity compositing due to this:
metacity fails if compositing is enabled.
(Source)
So is there anything else I can do to speed up the user interface and disable any type of effects?
First of all, you cannot use metacity and GNOME-Shell together :).
GNOME-Shell depends on Mutter, and if Mutter does not support turning down its effects, then neither does GNOME Shell. The best you can do then is ensure that you configure everything around it to be optimal for its performance.
Some tips: