There is some piece of truth on both sides, so you even might stop quarrelling.
It is true that glxgears
does some simple things that are not typical for a 3D application, so it can be considered not representative.
But, if the slowdown is too big then other applications may be impacted too.
(Nevertheless, it may happen that the glxgears fps increases, but you still see no benefit for real 3D applications.)
I don't want to repeat my post again, so click here