PS: here's my own VST host with optional sandboxing and pipelining. Both features are orthogonal. https://github.com/Spacechild1/vstplugin

Supercollider (scsynth) and Pure Data - both I'm contributing to -, have single threaded DSP. Users told me that they can use many more VST plugins when they enable the pipelining option in my extension, which matches your experience.

Again, I can't look at the source code for jBridge. The only thing I know for sure is that sandboxing itself only causes performance loss.