Try the speedtest app on the aTV, even though it measures to an outside server it can still help you diagnose. If the reported speed is way below what you expect to see from your provider, it means the wifi connection between your aTV and the router is not so good.
**Have you rebooted your router? and all network gear?
any time you start having network slowdowns, this should be the first step you try.
sometimes the multicast packets that airplay and a lot of other things on your network use can bog down a wireless router.
I have my wireless set to reboot every other day, because It starts to slow down. I'm using Unifi, which you see in corporate setups.
ISP provided routers have notoriously bad wifi to start with, and there is also the possibility it's starting to die.
I have a few 4k HDR movie files, the biggest of them are about 40GB, even at 40Mb a second, they will do a direct transfer in about 2 hours, meaning it should play fine, even at that low speed, since it can transfer the entire file in less time than it takes to play it.
**Have you moved anything in your setup or house?
router, aTV, TV, furniture?
My aTV is wired now, in the past I've had it on wireless, and mounted on the back of the TV, I know a couple times I had to move it, as I think I had it too close to the wifi in the TV, and they were causing problems for each other.
Try to imagine a straight line from your router to the aTV, through walls and anything else, if there's anything very dense or made of metal, you might want to try moving
You also don't say if your computer is wired or wireless. If you're wireless, the aTV and Mac both talk to the router, and not directly to each other. So if the Mac and aTV are close, but the router is 2 rooms away, your sending that video file over 2 poor wireless hops.
**have you tried separating your bands? (separate 2.4 and 5gHz names for your networks)
even though 5 is faster, it doesn't go through physical obstacles as well, So depending on your particular setup and home layout, your aTV might get faster signal with 2.4. And with separate names you can force the one you want. The automatic switching is pretty good in most situations, but not all.
AppleTV is horrible too, just not too many people pay attention.
or maybe other people aren't having problems.
of all my apple devices, the aTV is one of the most stable
my only complaint is the physical design of the remote.
my biggest disappointment is the HomePod.
with the latest software update this week, it's gone deaf. I've heard "I'm sorry, I didn't quite get that" about 5 times in the last 4 days. I might have heard that once or twice in all the time before the last update.
She also is cutting me off mid sentence a lot more. Where she'll just hear the first half of a command and act on that. even though I did't pause between words.