It does not look like a major speed up when developer himself write in the commit that he don’t perceive the improvement and that change is valid for a point size of 30 that we don"t really know to what it corresponds.
It is hard to make improvement on something that was not tested a lot, yet. It is easy to have modifier preview doing weird thing by changing way to draw background from Theme to Viewport. or Color Type from Material to Single.
I obtained this.
But it looks like Flat shading is a bit faster than default Studio.
We can let them stabilize viewport features before asking for speed.