Super interesting. Are you testing local render times or DR&BB? And total frame times (Scene start-up, Precomp, Render, Denoise, Save, close-scene) or just pure speed in terms of Rays per Seconds?
I know from my experience the higher-core and more complex (multi-CPU,etc..) machines can for strange reason struggle with non-rendering aspects, particularly pre-comp processes can wildly swing. For that reason I often avoid Distributed rendering on complex scenes as the DR could take as long as 10 minutes or never even start due to some random bug.