for me fast trainer is the 5* (late 5*, not like those new youngsters of 80%) player who can hit 1 sp at 3 bars match training , or + 4 skills in a set of drills with -18 condition loss (avg, after 4-5 training tests ).
So, this player at the end of the season can be near 7* without spending a bunch of greens to boost him.
* So, you can keep a high q player for 4 seasons

Was he fast trainer at his youth ? Probably, but not now.