I'd like you to try something out for me.
Pick one of your players who is very close to the next star or has enough SP to reach the next star and train him on all 3 training exercises. Record the skill gain values and then assign the SP to that player. After an hour, do the same and record the values again.
If there is no difference then assign all SP. If there is a difference, in that he was a faster trainer before gaining a star, then do not assign SP until you are finished.
This will prove that training speed is affected by unassigned SP