CPU rendering will be restricted to a BVH2, which is not ideal for raytracing
performance but can be shared with the GPU. Decoupled volume shading will be
disabled to match GPU sampling.
Thread priority or GPU sync tweaks are likely needed to improve performance,
but might as well post the patch for testing already. Perfect scaling is not
going to happen due to BVH2 usage though.
Go to User Preferences > System to enable the CPU to render alongside the GPU.