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).