Seems that cegy is right : there seems something wrong with the decoding (h264) from this file : in my benchmark I got about the same results.
Strange is that the decoding seems to be limited to 1 thread.
I tested with 12 threads and with 1 thread : same result !!
During the benchmark, I had a CPU load between 5-7 % (read from taskmanager), this should to be 50-70% or higher when testing with 12 threads.
Tested on i7-970@3.7/ 6+6 cores.