I just had a couple of LGB G Scale moguls converted to digital (Massoth XLS decoders) and I noticed they run slower than they did on an analog track. Is this a common phenomenon? I noticed the same happened when I added a decoder interface to one.
Yes, it is quite possible, and could be explained if the V-Max (V-High in my QSI manual) output of the decoder to the motor is less than what could be deliverd to the motor using your prior DC set-up. If your decoder can only send 15-20 volts itself, but your DC system topped out at 24 volts, you would undoubtedly notice a drop in top speed.
(I don't know the typical voltage ranges in our scale, so don't be put off by my numbers.)