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...