Generally speaking, it's soft shadows that can slow things down, but if you're not using it, then it could be anything...processor speed. There is a 'priority' control which can be set to low, medium or high, however, it's set at medium as default.

 One thing you can try is to stop the render, save the scene and then exit. The restart Bryce and reload the scene, and resume the render. This should clear the cache and give you a little more speed.