By default PPro renders previews to I-Frame MPEG but it outputs whatever you choose. However, you can set up a custom sequence to render previews to another codec if you choose. So say your footage is ProRes 422 and you’re going to output ProRes 422. You could set up your sequence to render previews to ProRes 422 and that way if you check the “Use Previews” when you output those frames will be used in your output thus saving time since the frames are already cached and consistent in the workflow. I don’t mess with that Maximum Render quality thing. The time and place to use it escapes me.
If you need to work in a lowrez environment on your timeline just select a different resolution on your Program Monitor. It’s like built in Proxy mode. I’ve been editing 5k R3D at 1/8 resolution with no problems at all on a 2013 Macbook Pro.