It's a Synology DS720+ with a Celeron J4125, 2GHz, 4 cores, 2GB of RAM.
I thought it would be fine, because Plex has no issues, but it was painful. Every client I tried on the AppleTV was equally painful, and I didn't even try using a client until making sure all the metadata was downloaded and setup via the web UI. I was very deliberate and did one thing at a time, spending a whole day on it (mostly waiting and browsing to different screens to force metadata to get downloaded and cached).
A Pi isn't a NAS with an old CPU and minimal RAM which is running a bunch of other things as well. I have run Plex on my Synology, and it's fine, but I also have 8gb ram and honestly always have a better experience running Jellyfin on a small PC. Easier that way, and you can run other services. Before the rampocalypse you could find mini-PCs with 32gb ram and 1tb SSD for like $350, and have great transcoding ability when you are on the go or have multiple streams going.
I thought it would be fine, because Plex has no issues, but it was painful. Every client I tried on the AppleTV was equally painful, and I didn't even try using a client until making sure all the metadata was downloaded and setup via the web UI. I was very deliberate and did one thing at a time, spending a whole day on it (mostly waiting and browsing to different screens to force metadata to get downloaded and cached).