Haven't noticed it here. What is your CPU model?
CPU Model is E5-1650 V4
I've saved out some less complex scenes today and tested the Image Editor, and it's lightning fast. It seems to be with just this one scene (Although I have yet to test another complex scene with 1.7 Image Editor). The only thing that has changed with this scene between 1.6 and 1.7 is a lighting model imported from Sketchup and the corresponding LightSelect pass. I also allowed the render to run for several hundred passes overnight, although I'm not sure if this would contribute to a slowdown.
I have just resumed the render from the CXR in Max, and there is no slowdown when using lightmix or adjusting and post processing options - an action that gives a 3 second hang in the Image Editor is instantaneous in the VFB.
I don't have the 1.6 Image Editor to test this with, can I get a download link?
I can also provide you with the .CXR and possibly the Max file if needed.