This is something that is quite well established, and anyone who has trained players knows this. In the game manual it says
Additionally, each player (active as of the start of season 5) has a potential which will determine the best that player can possibly become. Potential acts as a "soft" cap on ratings, meaning that a player who has trained to his full potential may still improve, but will improve much more slowly than a player who has not yet reached his potential.
If a player with higher potential were to train faster it would be mentioned here