Cycles GPU rendering slow AMD 7950 GCN-OSX 10.11.4-Mac Pro 5,1 Quad Core-24 GB RAM

I am rendering using OpenCL Cycles Blender 2.77.
Graphic card
My rendering process is slow and OS X is having problems to refresh my monitor.
There is no HDR images included.
Where the problem is ?
First image shows GPU render time 6:20.
Second image shows CPU render time 3:37.