Simply put, x264 HD Benchmark is a reproducible measure of fast your machine can encode a short HD-quality video clip into a high quality x264 video file. It's nice because everyone running it will use the same video clip and software. The video encoder (x264.exe) reports a fairly accurate internal benchmark (in frames per second) for each pass of the video encode and it also uses multi-core processors very efficiently. All these factors make this an ideal benchmark to compare different processors and systems to each other. Read more here.
In the first pass of X264 benchmark encoding tests, AMD’s A10-5800K ran 20 percent faster than A8-3870K. The second pass, which is much more demanding and CPU intensive, reduced the advantage to 7 percent.