...its false to say that FCPX is faster with h264 in 4K.
First off h264 is a delivery file and not for edit....
When scrubbing on an H264 4K timeline, the FCPX frame update rate is significantly more responsive than Premiere CC, running both on a 2015 top-spec iMac 27 with a Thunderbolt drive array. That is using 1/4 resolution playback on Premiere, GPU acceleration enabled and "Better Performance" on the iMac. Premiere's time lag to JKL input when going from FF to REV on that timeline is so sluggish it feels like the keyboard is broken.
The less efficient Premiere playback engine saturates the CPU during simple scrubbing, causing the fans to spin up.
Anybody can test this themselves. Just download FCPX and Premiere CC, import a H264 4K file using leave files in place on FCPX and drag the playhead back and forth, then use JKL and try going from FF to REV with each editor. On the same hardware, FCPX is generally much faster than Premiere for many common editing tasks.
E.g, FCPX exports H264 4K about five times faster than Premiere -- 00:29 vs 02:20 to export a 1 minute clip.
Re editing format, Adobe's long-held position is the Mercury Playback Engine is so fast that it's unnecessary to transcode to a mezzanine codec. In fact the Premiere CC induction video on Adobe web site says:
"...the 64-bit optimized Mercury Playback Engine...allows editors to work at...4K and beyond...without the need for time-consuming transcoding".
https://helpx.adobe.com/premiere-pr....html?set=premiere-pro--get-started--overview
Adobe now understands that is not always viable at 4K and beyond (at least without requiring huge hardware), so they are fortunately adding proxy support in an upcoming version of Premiere. That has been an integral part of FCPX for years.
Hollywood editor Vashi Nedomansky edits feature films using camera-native codecs in Premiere CC. He discussed his workflow in this talk:
https://images-tv.adobe.com/avp/vr/...f939e009_20151006111030.854x480at800_h264.mp4
Premiere is a great product, I have a CC subscription and have done extensive documentary editing on Premiere since CS5. It does OK with H264 1080p on a top-spec iMac but can be quite sluggish at 4K. If you're on a Windows PC and throw a lot of hardware at it, then H264 4K is handled better. Unfortunately that's not an option with most Macs.
The good news for Premiere users is Adobe is working to improve performance. OTOH Adobe committed to improving After Effects performance using the Metal API and demonstrated 8x improvement, but they have since backed off delivering that:
https://www.macrumors.com/2015/10/05/adobe-backpedals-metal-after-effects/