If you start with the fast skills first, you are finished earlier with them, and could train the slow one longer so that in the end you finished at the same time.
I'm not convinced
Let's say that our goal is that our player had to arrive to the same level in both a fast and slow skill(call them X and Y)
And let's say that I will train first slow skills and you will train first fast skill.
If our players always had the full training,there will be a point in an avdanced age where the sum of their X+Y skills will be equal,while they are still training,right?
If the point when X+Y is equal for both the player,arrive when they still not reach the goal,it means that the player that had to be completed in the X(fast) skill,will finish earlier than the player that has to be completed in the Y(slow) skill