Music generation can be query fast (millisecond range) or really slow (seconds range), depending on how complex the constraints are for the generated songs. Virtually all time for song generation is spent in finding a random song activity matrix that fulfills the given constraints. Once a valid matrix has been found, rendering the song is very fast.
As you can see from the simple example provided in the applet (http://www.soundhelix.com/applet/SoundHelix-applet.jnlp
), song rendering times can vary (timing information is logged in the debug output), but it's fast, because the constraints are not very restrictive.
After generating the song, the player (which is a MIDI-based player) takes almost zero CPU time; your MIDI playback device (whatever you use, for example the Windows MIDI device) might take some CPU time, but the player doesn't.
It's absolutely no problem to let it run in the background while playing a game (3D shooters, etc.), even if you use a software synthesizer. I'm sure about that, because I do it all the time. ;-)
I'm not saying that there is no way to use SoundHelix in commercial and/or closed source games, all I'm saying is that this is not allowed with the GPLv3. I'm open for negotiating non-free (paid) licenses for commercial and/or closed source games.