I mostly use Emby for older TV, so 4k hasn't been a large part of usage to date, but now that we're all shut-ins, I've been watching more movies. My setup has issues playing 4k movies; every few minutes play back will stop for a second or two, then continue. These come in clusters of three or four, aren't tied to specific points in the film, and it doesn't appear to be 'buffering' in the usual sense, in that pausing playback doesn't really affect when the next burst comes.
To narrow things down, I've dragging a wire down the stairs to eliminate WiFi as a factor (no change on GigE), and tested with both Infuse 6.3 and MRMC. Neither of them show the issue. Infuse's speed test showed 940Mb/s most of the time on the wire, with occasional dips to 250Mb/s or so. So, this points to the player app, I think.
I've not extensively tested file types and content, but these tend to be 4k mkvs, and the server claims direct streaming, with the comment that it's repackaging the stream. Typically there's tens of minutes of transcode buffer showing. Server load is light. (It should be, on a 6-core i7)
So, what should I try next? Infuse 6.3 could be OK, I guess, with some work. MRMC's UI leaves a lot to be desired. I wouldn't think remuxing the files will help much, since the other players don't have an issue.
Are there beta invites available? Anything else I should look at? Or just go get a Shield and be done with it?
Server: FreeNAS 11.3-U1, i7-3930K CPU @ 3.20GHz, 32GB RAM, 8x8TB RaidZ2, Emby 220.127.116.11, GigE, or 4xGigE LAG, makes no difference.
Client: Emby tvOS 1.4.4.07 on an AppleTV A1842 (32GB 4k), tvOS 13.3.1 (17K795). 802.11ac with tx/rx rates in excess of 300Mbs or GigE, makes no difference.