I think trying to get the best picture possible into the 15 minutes he can wring out of a day would be worthwhile as well, if his goal is really increased speed.
When he has the time, sure, do the more complicated ones, but I just think that days where he /doesn't/ have the time, where he's got external pressures, are a good excuse to really push his quickness (and time planning!) skills to the limit.
Ultimately its up to you though, The Fool, obviously, just a suggestion.
But there's definitely an art to drawing quickly, and it doesn't take reducing the quality to do it - though it may require reducing detail levels and changing the style a bit.