I have tried using Map based routing to get better turn by turn directions on courses, or using a previous activity as a course and it is oh so broken!
First, for a 20km course, it spends about 25 min "calculating" (and draining the battery significantly) while only displaying a very blurry map. I thought, ok, maybe these courses/tracks have way too many points and it tries to incrementally recalculate the route point by point. So I created a very simplified course, just approximating the track, with < 100 points. That yielded no significant improvement.
Also, if you're very patient and wait for those 25 minutes, my experience is that if you go off track, weird things happen and it does things like sending you back to the start or a previous point long ago and even if you go back on track it doesn't resume navigation and insists you go back home 
So for now I'll keep using navigate using course and not map, but I think there is an issue here as even for simple tracks it's completely broken.
