Can "Discrete only" GPU cause slowness?
The author of gfxCardStatus answered here.

If your goal is better performance regardless of power consumption,
then the solution is really simple: quit gfxCardStatus, delete it, and
disable automatic graphics switching in System Preferences &gt; Energy
Saver. That will cause the discrete GPU to be used all the time.
When gfxCardStatus is running, it manages the automatic graphics
switching setting for you. Dynamic Switching is equivalent to having
the automatic graphics switching checkbox checked.