Taking a look at your logs, it seems like your performance issues are due to not having your CPU available as an OpenCL device. You can use either the Intel OpenCL drivers, or the generic ones from AMD. Either should work fine.
https://software.intel.com/en-us/articles/opencl-drivers(Use 2.9.1, stable)
http://developer.amd.com/tools-and-sdks/opencl-zone/amd-accelerated-parallel-processing-app-sdk/Technical reasons: As counter intuitive as it may seem, newer GPUs actually have progressively worse performance at double precision. Things have shifted to focus on performance of floating point, since that's currently what's used for rendering. They instead produce cards with high Double precision performance, that are used for professional applications.
The problem, is that US² is pretty much exclusively in double precision. Modern CPUs have no issue with doubles and will often perform better then a fast, modern GPU.
Here's a concrete example, this is the processing power in both modes on a GTX 970. In (Giga Operations Per Second)
Single (float) 3494
Double 109
Or in short, almost 1/34th the performance.