$3,500 iMac 5K against an iPhone 12 mini

Final Cut uses GPU for encoding / exporting as well, I believe.

That's not necessarily an invalid comparison. From a technical perspective, NVIDIA's NVENC encoders for example have been able to compress H.264 with similar efficiency to x264's slow preset at several times the speed for a few years now.

And there's also the fact that for many purposes, it simply doesn't matter. Filmmakers often don't want an impeccably compressed, low-bitrate output ready for streaming directly, they want an output with the highest quality they can get, as fast as they can get it and don't really care how high the bitrate is as long as it's reasonable.

In use-cases like that, a faster render/encode is what's valuable and it doesn't really matter which processor is used to produce it.

Same resulting file-size, supposedly same settings

(Mentioned further down the twitter thread)

Looks like Final Cut to me, which is Apple's software.

That’s one step closer, but it all depends on what exactly you want to test and compare. If you want to test raw speed you probably need to use comparable transcoders.