I asked Marin about the hidden skill "training speed". He said it is a possible hidden skill, but we have to find out if it's actually one.
I know, a big help... But by tracking the training of over 400 players over the last 15 seasons, there are clear candidates which have this hidden skill. There is no other explanation which would make sense.
If a player trains with a lv 5 coach faster than another one who has a lv 7 and is 2 years younger (same training, same minutes, almost same height) than there has to be such a skill.
And than there is the negative example. Some players are soo slow. Faultier, a former U21 coach of Germany, has such a player. His trainee trains almost 1 week slower than the "normal" one.