I have news. I normally use my old iMac as an external monitor with my macbook pro, connecting it via the firewire port and adapter.

I had to use the SwitchResX app to get a decent screen size and experienced only very sluggish performance.

Today i discovered that you can get far better results with native macOs stuff. Doing:

System Settings -> Displays -> Show resolutions as list -> Show all resolutions

works like a charm, with a lag that is very acceptable. I have no idea when this feature showed up.

It has been a long wait, but thank you apple.