There seem to be too many misconceptions here concerning matroska.
First: mkv is a container and most problems stem from an incompatibility of the contained content with your software/hardware not the container itself.
-mkv does not cause your computer to lag, H264 does and only in HD resolutions (720p, 1080p).
-mkv does not cause subs to be displayed improperly, that's your inferior softsub renderer (such as the one in VLC)
-mkv is not the cause that no subs appear at all or the audio is in English, that's your inferior or missing mkv-splitter
-mkv does not make files bigger, that's the decision of the subbers to limit mkv to HQ H264. In fact, the same video in mkv as in avi or mp4 is smaller by a few 100 KB
The solution for almost all problems is the CCCP and the included players. If you can't use CCCP (because you're on a non Windows OS or need it portable) you have to use Mplayer but you will be missing out on some of the advantages.
If your computer has problems with H264, use CoreAVC. If even that fails, get a better PC.