After upgrading to Mojave 10.14.1 and FCPX 10.4.4, I did a few basic encoding tests, then upgraded to Pro Video Formats 2.0.7 (previously was on 2.0.6, I think).
For certain codecs and especially for ProRes 422 output, I'm seeing a significant improvement in encoding performance after the 2.0.7 update. I tested the same sequence on my 2017 iMac 27 and don't see any difference, so it seems isolated to the iMac Pro.
It's possible I made a mistake but I can't easily revert to the previous Pro Video Format to double check it.
If anyone is running Mojave and FCPX on an iMac Pro and if you have *not* yet upgraded to Pro Video Formats 2.0.7, you might run some encoding tests before and after the 2.0.7 update and see if you notice anything similar.
The encoding speed is due to Compressor being 64 bit now. All Macs will see Compressor (and FCPX to an extent) encode files faster. At our TV station our MPEG encodes for broadcast take literally half the time they used to. Pro Video Format introduces nothing much that would change encode speed. But the fact we now have a 64 bit encoding engine means a lot.
I'm using an iMac Pro and tested upon upgrade. Speed increases came with Compressor update, not with PVF update.
In attempting to isolate this I manually rolled back to PVF 2.0.6 by restoring the files from a Time Machine backup to ~/Library/Quicktime and ~/Library/Video/Professional Video Workflow Plug-Ins
It didn't completely restore all previous behaviors but it reverted the previous super-fast iMac Pro behavior on ProRes 422 export -- even though I was still running FCPX 10.4.4 and Mojave 10.14.1.
Examination of the ~/Library/Quicktime and ~/Library/Video/Professional Video Workflow Plug-Ins shows that most of the codecs are changed by the PVF 2.0.7 update.
The most interesting behavioral change is the hyper-fast ProRes 422 output only happens (1) On the iMac Pro, (2) If the timeline is fully rendered and (3) Source media is also ProRes 422.
There has always been a question of why FCPX re-encoded fully-rendered ProRes 422 material if the output codec was also ProRes 422. It should be able to just copy the render files which by default are ProRes 422. On the iMac Pro running FCPX 10.4.4 and PVF 2.0.7 that's now what it does. The I/O rates total about 1 gigabyte per second. This only works if the output is the same as the render format, ProRes 422. Picking 422 LT or 4444 output causes re-encoding (as expected).
For reasons I don't understand it doesn't do this on my 2017 iMac 27, even though it's also running Mojave 10.14.1, FCPX 10.4.4 and PVF 2.0.7. It appears to re-encode the ProRes 422 output.
On the iMP the super-fast ProRes 422 output for pre-rendered timelines happens for most effects I've tested, but not for stabilization. I don't know why.
It also doesn't show the fast ProRes 422 output behavior if the media is H264, even though the timeline is fully rendered (which means it exists as cached ProRes 422 files). However it also doesn't happen when using 4444 media, 4444 render files and output. It seems 422 is a special case or else there's some kind of performance bug in the other code paths.
Another interesting behavior is the iMac Pro's 4k H.264 "Fast" encoding performance has improved. It was formerly slower than the 2017 iMac. I filed a bug on this months ago. The iMP was already faster on H264 1080p Fast and HQ output, also 4k HQ output; the slower iMP 4k H264 "Fast" encoding was an anomaly which seems now corrected.
However the iMac Pro's 4k H264 *decoding* performance has not improved -- it's still more laggy than the 2017 i7 iMac on fast forward and reverse operations on the timeline.
Whether the source is Mojave, Compressor, FCPX 10.4.4 or PVF 2.07, at least there are some FCPX performance improvements on the iMP.