I'd be incredibly surprised if mobile power management did not account for the difference between "blitting array of 2D pixels" and "running massively parallel shader". Even if not done for energy conservation, it might be for TDP profile.

Separately, it's long been a thing on Nvidia Optimus laptops where the most inexplicable thing could cause the iGPU to be swapped out for the power-hungry discrete GPU. A GL-powered terminal definitely seems like it could be in that territory.

In short, many reasons it could be less efficient, hence the question, is there any evidence that it /is/ less efficient?