Awesome, that fix worked... well, I did it slightly differently, I used:
sudo chmod a+w /usr/X11/var/cache/fontconfig
The first time I loaded an MKV after that it took 1 minute to load, but generated the cache info it needs to go back to loading files with subtitles at normal speed. I hope you guys will do a proper fix for the next release, but at least it all works with this little workaround.