Unless the training is changed in the upcoming 2019 release the quickest way to get your training level up is to make sure you have the maximum number of players in your squad. All 28! Reserves can be 3 star 1 token buys who will never play. Make sure you train them to the full 48% every day. 6% gain every 3 hours. 24 hours in a day divided by a gain every 3 hours = 8 gains of 6% every day. 48%.
I train all my reserves and the subs who have no chance of playing down to 51% first thing every morning and then forget about them until the next day.