Robert, you may well hate the "Reaper runs fine on my system" remarks.... but if someone is running what appears to be the same system without the problems you describe, then it may help to narrow down where the problem is.
For instance, as I posted in the other thread you've started on the same topic, I have a very similar vintage of iMac, also 5k screen, and also recently updated to Sierra. I don't see the laggy response you do, and find Reaper perfectly workable on it.
I don't say this to annoy you. I say it to try and help you pin down what is DIFFERENT on your setup and causing the problem, since it appears not to be universal to iMacs, or 5k retina displays, or OS Sierra. If you would like me to give more details of the exact config of my system, please say and I'll try to oblige.
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