My 3.5 year old boy is very well potty trained at this point, though he still usually needs us to tell him to go to the potty (he likes to hold it in a long time). He is capable of going completely by himself and prefers it this way. However, when he goes potty, he spends a long time either just sitting on the toilet after he's done with his business, or (after getting off the toilet after we tell him to) just laying on the floor in the bathroom. It often takes him 15 minutes to potty and wash his hands, even with us following up with him several times while he's in the bathroom.
While in the bathroom, when we follow-up with him to encourage him to wrap things up, he often yells at us and gets upset. And it's not like he's busy doing something: sometimes he is just laying on the floor, doing nothing.
This happens whether it is #1 or #2. In fact, times in the bathroom when he goes #2 can be faster because he's more excited about that achievement than #1.
What can we do to get his time spent in the bathroom down to a reasonable amount of time?