Again, sorry for the infrequent updates, couldn't get anything done most days last week. Since Zut mentioned something relevant to it, I guess now a good time for some lore on shifting/magical energy stuff!
As Zut says, they can't shift often, and can probably only manage that form for a few minutes at most, or two or three times a day in quick bursts. The initial shift uses up more energy than maintining the form. The imps and dragons are both inately magical creatures, but the dragons better potential in that regard. While the imps regenerate theirs with a bit of rest, the dragons are basically always on, plus can generate even more from sources like food (Tan's frequent shifting and subsequent appetite being a big reason she got some new cow skull decorations for her cave a few chapter back!)