It seems to be any .TS file, whether it's mpeg2 or h.264. Any .ts files when played from within MP just stutter like crazy and the whole thing becomes slow to respond. If I quit MP and just play through media player then they play back fine as expected. This was never an issue before so something has changed with MP as I haven't changed anything on my system, hardware or software wise.
The problem goes away if I rename any of the files to .mkv instead.
The problem goes away if I rename any of the files to .mkv instead.