Fast/slow trainers

[Fail G]

There's a lot of talk about players who are fast or slow trainers, and clearly faster trainers are more beneficial than slower ones; but I'm wondering how fast a players growth can be?

Here's a print screen of my team after everyone did a normal intensity stretching training session. Obviously  dos Santos Moura is the clubs fastest trainer, and S El Quabbani is the slowest, and it's no surprise with those two considering the 12 year age gap, but C de Oliveira Costa is the youngest and he's a fairly slow trainer.

I think I heard somewhere that the players quality also affects their training speed, but again I'm not too sure how it works.

What's the highest growth you've seen of a player, and how do you think this could be improved?

Cheers!



Edit:
Side-note, ignore the horrible formation that I've currently set up; I've already won the league so I'm just playing the players with the least morale to boost it a bit