I am a new user of CxD, and am considering purchasing it for my .mkv to DVD needs. I have a dual Opteron 275 with 2.5GB of RAM. Setting the nice level of CxD to normal and starting a conversion session makes my system become almost completely non-responsive. This is even though Task Manager says CxD is only taking up one core and my system is at ~30% usage.

What gives? Also, why isn't CxD multi-threaded? Or rather, why doesn't it fire off a separate process for each file that is being converted?

If it matters, this is with two 1.07GB .mkv files that are MPEG4/AC3, using CxD v2.1.14.223.

Multithreading is not so easy to do, and it would be useful only when converting multiple DVDs at same time.

Your problem however is very odd.
What happens if you set CxD below normal?
Non-responsive system is usually happening on slow systems or when priority is higher.

what is the play time on these over 1 gig mkv files?

~52 Minutes.

Partially true. It could also be used when converting multiple titles/episodes for a single disc. The simplest solution would be to use a conversion process (not thread) for each episode.

I am planning on doing this for my next conversion. I'll report back.

Setting the nice level to "below normal" returned the machine to a useful state. Again, Task Manager was showing about the same 30% CPU usage, with one core monopolized. It must be something evil CxD does to Explorer to make it become unresponsive.

How long does it take to convert those 2x52 minute files?
If they finish in total just under 1 hour then CPU usage is not correct.
With that amount of RAM and that CPU it should finish in roughly 40 minutes to 1 hour, depending on amount of subtitles etc. on originals.