The speed in which a player trains decreases as he jumped up to the next star.

My star player is 9 star. At the start of every season I power train him right up to ten star and because of his high rating I only get around 10% improvement for every high intensity game he plays in training. It costs me around 500 Greens to get from being low 9 star to the verge of 10 star now. The same player when I got him through my academy was getting over 100% improvement in training each time.