I assume you are asking about data rate (not propagation speed, and not signal speed which is a term that doesn't mean anything). But clarify if you are asking about propagation speed and whether the return current causes the single-ended signal to take twice as long to register as the differential signal. I don't think it does but I can't explain it well. The answer below is about data rate only, not the speed of propagation of the electrical signal down the wire.
For the same slew rate and voltage transition levels, a differential pair has double the noise margin of a single-ended signal.
That means that you can reduce the voltage transitions (and associated logic thresholds) on the differential pair and reduce the noise margin until it is then again equal to the single-ended. The result is that the the slew rate has remained the same, the noise margin has remained the same, but the voltage transition on each wire has been reduced so when you flip the signal on each wire, it is able to finish its transition faster which lets you send data at a higher rate.