As part of my Beads work, I explored the possibility of bypassing Javasound and working directly with a low-level sound library. In doing so I built an (almost) complete Java wrapper around RtAudio, a low-level, cross-platform audio programming library. My project, JRtAudio is now accessible online, so check it out if you're interested.
As for whether Javasound is slower than, say JRtAudio, the results are not looking good, but are kind of obvious. The benefit of using JRtAudio only appears when you want a really simple API combined with low latency.