I see good performance on a mid-2014 MacBook Pro (with nVidia graphics), and poor performance on a mid-2015 MacBook Pro (with AMD graphics; otherwise identicall

I see good performance on a mid-2014 MacBook Pro (with nVidia graphics), and poor performance on a mid-2015 MacBook Pro (with AMD graphics; otherwise identically configured). So, yeah, looks like some kind of issue with Apple's AMD graphics driver.