Yes, 15mph is not the theoretical max speed of a bike.
For me, on my $300 Trek hybrid, traveling on flat land, that's about as fast as it can go.
I'm sure someone in good shape on a nice road bike can go faster.
For a cyclist sitting upright (CdA of .5) but with good tires and pavement (Crr of .0035), it takes about 120W to hold 15mph on flat terrain. I would agree that this is about the limit for most casual riders. Serious riders with more power typically don't have a problem riding around other bike lane users. What I'm saying is, I don't think the robots are burdensome for the average bike lane user, as far as speed is concerned.