@Attitude I obviously went about the resizing in the wrong way , so I did what you suggested instead. The appearance was not bad (after all, a lot of developers offer so-called lo-res liveries), but it made absolutely no difference whatsoever to the frame rate. It merely depleted the size of the loaded textures a bit.
Your point about different developers is, naturally, appreciated. Variety is the spice of life!
I'm on an iMac, which was top of the line (27", 4GHz i7, 32GB ram, 4GB graphics), but it's almost certainly the graphics card that is the weak link in the chain. Newer cards will surely perform better in Metal , and the latest iMacs have several other areas of improvement. I'll see when my new one arrives.
It doesn't matter how much I dial down the settings in b17, running in Metal. The frame rate is still poor. For this setup, OpenGL works better. It's also quite likely that Vulkan will run faster, and I could try this in Boot Camp..........
For me, it's mainly just curiosity, and it's not a complaint. Sometimes, these inconsistencies can easily be ironed out, but I think horsepower is the answer here. X-Plane is very resource hungry, and it makes Macs run quite hot.
It will be interesting to learn how MSFS 2020 runs on modest rigs, as nearly every tester seems to have been equipped with top of the line CPUs, and similarly expensive 11GB 2080Ti graphics cards!
Thanks for your interest and support.