Inconsistent Transcoding Issues

I am running plex on an UnRAID Server with 16GB RAM and an i5-8400. While on my local network (which has been able to wirelessly transfer files at 40-70 MBps), I am having issues streaming a 720p video file (4.4GB) to the plex website on mac and windows both and to the iPhone Plex app. After running for a little bit, I get the swirling symbol. Have also gotten a message saying that my connection to the server is too slow to stream the content. However, when I stream a 1080p file (20GB), it works just fine. Goes smoothly and doesn’t take any extra time. The 4.4GB file is an MP4 and the 20GB file is an MKV if that makes any difference. I don’t think it’s the hardware or my connection that is bottlenecking based on how well it did with the 1080p file, which makes me think it’s plex having issue with transcoding this specific file.

Any insight into this issue or potential ways to fix it?

is there transcoding during the 720p playback? (check plex.tv and click the status symbol, so you can see if it direct plays or transcodes)
anyway, 8400 should transcode fine a 720p video, so seems weired
do you know the video codec? mp4/mkv is only the container. (use mediainfo)