I was demoing Aaltoverb and I noticed the CPU usage was really high when the GUI was open. Roughly 10% open and negligible when closed. GPU was higher than I expected for what is being rendered, around 20% on my card.
Are you using JUCE by any chance? I've read in the past their OpenGL wrapper isn't very efficient. Not sure if it's any better these days, I haven't kept up to date.
I'm using JUCE but not for drawing. Mainly just for low-level plugin support.
Drawing in Aaltoverb is all done on the GPU and not really optimized at all yet. So I'm not surprised to see a lot of GPU activity. I don't know what the CPU activity would be about. I'll be doing some profiling in the future, meanwhile I hope it still works OK for you.