Larry Jordan ran these tests in early 2015. I believe Compressor may have improved since then with the 4.3 update as well.
Note how Compressor compares to Handbrake on the 21″ iMac. But note how HandBrake compares to the MacPro.
Here’s the article.
Larry notes:
If you own a new Mac Pro, HandBrake is the leader by a wide margin, followed by Apple Compressor.
If you own an iMac or MacBook Pro, Apple Compressor is fastest, with HandBrake second.
