Thanks for the help so far! I do have a g-sync monitor, and tried disabling that. It does seem a bit smoother, but still not quite as smooth as when I'm on my other machine running the 60hz TV..
The PC w/ the issues is no slouch.. Modern CPU, lots of memory, 2070super, nvme drive. It's a two monitor machine, 1st is the gsync, ultra-wide at 3440x1440, and the 2nd is a 4k 3840x2160.
Knowing what I know now about the rendering approach, I decided to just try cranking it up to 200hz (even though I don't have a 200hz monitor), and it's buttery smooth now. I re-enabled gsync, kept it at 200hz, and it's still quite good.
So, I guess that's a workable solution, just crank it as high as it can go. That said, is there an open issue for the gsync bug? Feels like that would be good to address regardless. And I'm willing to bet I'm not the only one who's been thrown for a loop on this.. I get you don't want to tax the CPU on older machines, but I've been using Launchbox/BigBox for years now and didn't know that this was a way to 'fix' the performance... I figured the refresh rate was supposed to try and match your monitor, and the 60hz=optimal setting was also a bit confusing. Not sure what the right solution is.. you probably wouldn't want to default to 200hz for everyone, but it is such an improvement in my experience having tried it I bet others could benefit.
Thanks for the suggestions!