If the PMS is using the TSMuxer engine (it should for compatible MKVs) quality should be identical to mkv2vob.
Both of these methods are a simple Re-Mux (extract elementary streams, put in a new container, DON'T change the data). As such, no quality is lost and very little CPU usage is required.
I would check your settings because PMS should be able to remux and stream any file that is supported by mkv2vob even on a very old machine. Verify that you have tsMuxer enabled as I think you might be trying to transcode when you don't need to..