Windows 7 64-bit
daily built 11/3/2014, says unknown hash
Short description of error
1.) Render times with deformation blur seem to have skyrocketed recently with shape keyed objects.
2.) A bug I'm unable to recreate regularly where high shutter speeds create black motion trails rather than correct colors. Changing shutter speed and re-rendering has no effect, you must turn off motion blur, do another render, cancel, and then turn it back on in order to try another shutter speed value. Very weird bug.
Sorry I can't provide a file, it really seems to be hit or miss whether or not this happens, although the slowdown is reproducible. The scene I was working on was (essentially) nothing more than a shape keyed torus with an emission material and shutter speeds ranging from .05 to .5, so that might be a good place to start.