Thanks @CaseySJ for Gold Build! All just works.
I have one q, Front Panel USB 3.1 ports works at "Up to 480 Mb/s" only.
I understand my front ports can handle only 3.0 protocol, but 480Mb/s is it just USB 2.0
Your system use old Intel® HD Graphics 4000, all new Apple hardware use UHD 630.
I guess Intel® HD Graphics 4000 can't encode-decode 4k video etc, Mac OS use UHD 630 for graphical acceleration for H264 and HEVC. And don't use your 5700.
Try SMBIOS iMac Pro.
I guess it is depends on OS and FCP X versions, I run 10.15.2 and last FCP X 10.4.8
iMac 18.x SMBIOS use my UHD630 + 5700XT and Burce X is 10 sec, also I have 1h video in FCP X without crazy effects, rendering to H264 takes 30 min
iMacPro SYMBIOS don't use my UHD630 and Bruce X is 7,3 sec, same...