You're still going to be fighting with Retina. I had the same issues you do back in 2015 when I upgraded to a MBP -- the results of that are logged in these forums.
End result was that I switched to Mini vMac II for 68020 emulation, before isolating screen resolution as the cause. I set my screens to 1280x720 for video capture reasons, and that does not work in BII at millions of colors. 640x480, 800x600, 1024x768 and 1600x1200 all work just fine.
This will affect your tests in viewtopic.php?f=6&t=9282
Using BII at the standard resolutions, it performed similar tasks MUCH faster on my 2015 MBP. Same goes for if I dropped to 256 colors. Seems to be a BII bug (or two).