sample_decode_x11 cpu usage

Hi, 

I  run sample_decode_x11 to decode a 1080P H264 stream with 25 fps and with -r option.  It shows 92.7% cpu usage. My

computer  cpu information : Intel(R) Core(TM) i5-4590 CPU @ 3.30GHZ.
 

Why does it have such high cpu usage. Which part consumes such high cpu usage.

Thanks!