Things that affect training speed are:
1. Age ( The younger the faster )
2. Height ( eg: Taller players train RB faster than shorter ones / Shorter players train OD faster than taller ones )
Potential is something like a limit as to how good/how much training can your player receive, the higher the potential, the player can be trained to a higher degree.
Potential only causes training to slow down/stop only when your player caps his potential.
You can manually calculate the rough estimates of the cap of the potential using Josef Ka's formula:
(188524.1)or you could use the tools>salary calculator in
http://www.buzzer-manager.com to do it for you