Plus, pushing 15W through a ’351 on a thin pcb resting only on a little ledge would be cooking the crap out of the poor critter. Surprised it didn’t burn blue at that point.
So yeah, pent-up heat being so high, of course efficiency will be down in the toilet.
Limit it to 2A tops (I’d be happier with 1.4A) and it should be much better.