All of my teachers recommended one goes through all speeds, and that going slowest while maintaining all the requirements to the utmost is the mark good gongfu. Even my current southern mantis teacher said something similar about Som Bo Gin.
I train all speeds. I have timed my slowest possible Pi Quan, performed without stopping, to be at 2 minutes - just one side! Usually, once or twice per session I would go that slow. At that speed, a Taiji form could have easily taken me over an hour to complete. Sometimes I practice Xing Yi forms (which are relatively short) so slow they can take me 10+ minutes to finish. Usually the way I would practice a form in my Xing Yi is as follows: 2 repetitions at 'walking/casual pace' (the speed most people practice in), 2 repetitions slow (or very slow), 2 repetitions very fast. When I practice a single movement or combination, I would segment the training according to speeds. This also helps because sometimes you're too tired to go full speed, other times too energetic to do things too slow.