I have a new Imac 5k 2019 i9 and the Vega GPU 32Gb RAM but trying to scrub media files from my Panasonic GH-5 does not work. Cant just get a soft play function when scrubbing. The mediafiles are 4k 10bit 4.2.2. Converting to optimized works OK with the irritating difference that start play is delayed a second. this is not the case with the GH5 material. it starts playing immediately, it plays Ok but as said scrubbing is jerky. Thought that this new machine could handle this.
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In general, smooth low-latency scrubbing of 4k H264 can be difficult on most machines using most NLEs. Some H264 codecs are worse than others. Sony's 100 megabit 8-bit 4:2:0 XAVC-S is particularly sluggish -- even on my 2017 top-spec iMac 27 or my 10-core Vega 64 iMac Pro.
By contrast 100 mbps 4k H264 4:2:0 from a GH5 is smoother. The 400 mbps 10-bit 4:2:2 version is more sluggish. That one is particularly difficult since it won't play in Finder using Quick Look or Quicktime 10. It will play with VLC or after import to FCPX.
The difference may be that XAVC-S and the Panasonic 10-bit 4:2:2 codecs use variable-length GOPs, whereas the Panasonic 8-bit 4:2:0 stuff uses fixed-length GOPs which can apparently be decoded faster.
Similar 4k 300 mbps 4:2:2 material from a Canon XC15 is also H264 but uses an all-intra codec which is faster to decode. That or similar material is fast but it takes more space.
This isn't a limitation of FCPX, DaVinci Resolve is the same way and Premiere is much worse.
In FCPX the skimmer within the Event Browser is more responsive. As a workaround you can skim by mouse/trackpad and mark ranges. JKL operations in either Event Browser or timeline can be quite sluggish.
My documentary team usually transcodes all 4k H264 to proxies or more recently we just use 4k ProRes acquisition. That takes a lot more storage but does not require proxies to edit.