CPU usage is not a good way to benchmark for performance. There is some known problem that may increase CPU usage.
Please check this article:
I also suggest you to use \audio-video-codecs\application\umc_h264_dec_con application to benchmark the H.264 performance. It provides more accurate performance data.
I do not have FFMPEG data at hand now, but the result is something unexpected for me.
When you test with umc_h264_dec_con application, it just wants to make sure your test get the correct performance with IPP UMC sample code.
umc_h264_dec_con measure the the maxum framerate that the decoder can achieve. If you want to measure the one threading performance, set threading to 1 to run the application. For example, the H.264 decoder can achieve 120 fps. Suppose the frame rate is 30. so it only needs possibly about 25% ( 30/120) CPU resource ( one core).