In a full-rudder sideslip in a Schweizer 2-22 or 2-33 glider, is the descent rate higher at 50 mph airspeed than at 60 mph airspeed?
Full disclosure: I have exactly 3 hours logged in a glider and have never specifically flown the 2-22 or 2-33.
That said, I do have some experience and understanding of slips, and the principles should be the same.To answer your question, I will draw from quotes taken from this article on Boldmethod.com.

In a forward slip, the amount of slip, and therefore the sink rate, is determined by the bank angle. The steeper the bank is, the steeper the descent.

However, there will be a limit to how much bank angle you can introduce before you can no longer hold a constant ground track.

In light airplanes, the steepness of forward slips and sideslips is limited by the amount of available rudder, or the rudder-limit. You may reach a point where full rudder is required to maintain heading.

If you are already at this point, with full rudder deflection, maximum allowable bank, and a constant ground track, there’s only one variable left - Pitch. Pitching down increases speed and increases descent rate.

As you lower the nose, airspeed and descent rate will both increase. The additional airspeed will give your rudder more effectiveness, allowing you to increase the slip. The opposite is true of raising the nose.

This means that you are absolutely correct. Properly configured, a higher airspeed should give you a higher descent rate.