Which GPU are you using? I have a 2015 Mac Pro with two D500, before I had a mid tear iMac. The mac pro stands directly besides my "beast" a i7 6700, titan x, nvm custom machine.
I can't really describe it, but it's bothering me and I would put my hand in a fire betting that it's somehow measurable. It feels like the input lag of a cheap IPS-Monitor. Every klick, every slide, every window resize has this minimum lag of maybe 10 to 50 ms, worse on linux. That's hardware independent, because that was already bothering me with a MacBook Pro and my old iMac. I haven't booted OSX in years to work production because of that tbh (Every piece of apple hardware I own is booted 99% of the time with Arch or W10E) but did it just now and compared it side to side. It's still there. It's driving me crazy. I know that I am hyper sensitive for lags and stuttering because of my former quake carrier.