Well, I rebooted my modem and router and it didn‘t solve it. I also re-seated the Ethernet cable. When I set up the new box, I did the thing where I brought my iPhone close to it and set it up that way. It synced all the apps i have on my Apple TV’s.
Well, I'd do the wireless test before blowing it away (as that will take some time).
If changing to wireless eliminates the issue, then the issue is isolated to Ethernet (and, in all likelihood, the ATV is using the same chipset for Wireless and Wired networking). In that case, I'd try a known-good cable, directly plug the ATV into your router (eliminating all of the cabling in between). If the performance problem goes away, then you have a problem in your cabling between.
If that didn't fix the issue, and you want to try eliminating your configuration from the equation, you do a factory reset of your ATV - (you go into Settings, System, Reset)
I'd also select Reset and Update if possible. (did your ATV do a TVOS update as part of the initial setup?)
- then set the ATV up as a new device - do NOT use your iPhone to migrate your data, do not enable one screen - I'd also use a different Apple ID just to make sure (create a new one if you need to). This eliminates the potential for your settings being the culprit.
(You could at this point - or anytime before decide to contact Apple support)

They're likely to take you through similar steps though.
If setting up the ATV as a new device with a different Apple ID doesn't resolve the problem, then there's only two real potential problems left. You have corrupt TVOS (there should be instructions out there to get the ATV4k into DFU mode to be able to blast a fresh, downloaded TVOS onto it from your Mac or Windows box) or you have defective hardware in your ATV4k.