If you're just power training one player, you won't get the bonusses anyway.
forgot to say also about the gaining for the level of the drills.
Power training one player, can also add to drill levels.

But if those drills are mostly easy or very easy ones, it could take a long time to get your players to level up.
yes, time is important too
My main team is training lv34 and has 6 drill windows.
In which level do we have 7 ?