MacBook Pro 15" 2015 iGPU slows down from normal 750mhz to 200mhz
The new 10.14.6 macOS update says it "resolves a graphics issue that may occur when waking from sleep". I'm hoping that might be referring to this issue (I filed an issue a few weeks ago at https://feedbackassistant.apple.com/, so that may have prompted them to look into it, though they haven't responded to it).

I'll update this answer if I see the problem again after the update.

UPDATE: I'm still seing the problem in 10.14.6 (as well as OP who commented below), so this is unfortunately not the solution.