In the past, I have had no problems with video. Recently, I have horrible performance. Also recently, for unknown reasons, I lost my connections to the servers and had to do the setups again. Also lost Plex which I use only for home movies.
Now, I get frequent “whirlybirds” on mp4’s.
We had guests last night and all settled into watching a movie and we all gave up after 1 minute of movie and 10 minutes of black screens with a twirling circle.
WiFi or wired connection to the ATV? If WiFi trying an Ethernet cable even just temporarily for testing will tell you if your WiFi connection quality may have changed.
I’ll add one more thing, if you have the ability to change out the Ethernet cables even just temporarily give that a shot. I had an Ethernet cable go bad and not be a complete failure, just cut the speed to about 10% of what it was. That took forever to find and as soon as the new cable was in place everything was back to normal.