Here is the mediainfo (is there a better way to present it here?): MKVtoolnix shows 5 entires under global tags and then two separate 'tags' with 1 entry and 2 entries respectively. While I'm happy that they play in PLEX, I'm worried that there's something wrong with the files. My PC can play native 4k UHD files with no problem, so I doubt it's my specs that can't handle a 2gb 1080p mkv with around 1000kbps bitrate. When watching through PLEX on my TV (direct play - absolutely no transcoding), they play flawlessly. Weirdly, the stuttering is only present when watching the files directly from my PC.

Whilst this has worked with the majority of my library so far (and they load instantly on my TV - yey), for a lot of other files it has introduced massive stuttering. This quickly gives me an mkv contained version of the same file with supposedly no changes to quality. The video mode is 'copy' and the audio mode is also 'copy'. I'm using the latest version of XMedia, selecting matroska as the format and mkv as the file extension. A few people suggested I use XMedia Recode to convert my mp4s to mkvs (as my TV loads the mkvs far faster in PLEX).

Xmedia recode lossless 10bit to 8bit