I'm sure he can and does. But at a distance of 150 miles across mostly flat ground? In order to cover that in minutes rather than hours, he'd have to amp it by a factor of 60. So the max (downhill) speed of pyroclastic flow at 50 mph would increase to 3000 mph, or Mach 3.9.Morgoth can amp it up, it does not have be *totally* like a real volcano. and, affter all, we are talking about Thangoridrim here
The issue isn't merely the real speed of natural phenomena, but the speed which the viewer will accept onscreen.
This is definitely something we took time to consider in previous conversations, and in the end is only a part of the reason to show the initiation of the battle in the final moments of E11 and start E12 with the fires already having reached Dorthonion.