Vincent Laforet's blog post shows off the H264 performance and it's twice as fast as the current iMac for generating Optimised Media (converting to ProRes), which is really, really nice, but he doesn't go into how good the performance is in terms of transcoding to Proxy Media - just that they all performed seemingly similarly well.
Straight from his blog post:
"I imported 10.96 GB of DJI Mavic PRO 4K, H.264, 23.98 fps footage, a total duration of 24 minutes 16 seconds of footage transcoded to Proxy ProRes422.
All 3 machines created low resolution Proxy footage in under 8 minutes which was impressive.
I then had the software render ProRes422 Optimized media from the H.264 source media:
iMac Pro – 7minutes 56 seconds
iMac – 15 minutes 47 seconds
MacBookPro 15” – 19 minutes 55 seconds"
Source
Therefore, I'm uncertain as to how well it all performs, but I may switch my workflow to using Optimised Media if the transcoding is that fast.