Some updates posted at http://code.google.com/p/klystrack/ including a new OSX binary (thanks Deltafire)!
I did some updates on the rendering and also the synth that should cut down CPU usage but only noticed a minimal speedup. I'll add an option to disable oversampling in the synth which should make it a bit more usable on slower computers (high notes will sound worse like they did on some older builds). That might be the thing that happened that older build and the latest build.
I also experimented with OpenGL rendering with this library that should queue stuff properly but I noticed hardly any difference (still slow when the screen is full of patterns). So I guess this has to do for now. But I can still do something about the synth if it is too sluggish.