Is there any indication of the energy impact using a GL context to render plain old text? These concepts seem incompatible in the context of a laptop, and who needs GL here when the bottleneck is biological. One of my favourite working modes is shutting down everything and just hacking away in vim, partly for distraction management but primarily because of battery.
Why would GL rendering be any less efficient ? You don't have to rerender the entire screen in GL or render at MAX FPS if nothing changed.
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?