Stu Wart wrote: ...Same Mojave version, (10.14.6) exactly same Fcp version (10.4.10) and BruceX is taking half the time on one on them !...
If the exact same procedure is not followed, sometimes BruceX can have widely varying run times. Examples:
(1) Background rendering should be disabled, otherwise export time may vary based on how long it auto-rendered. If testing two systems and only one has background rendering disabled, this could make a big difference.
(2) Export should be to ProRes 422 not H264 or any other codec. Otherwise it is testing encode performance (not render performance) which can vary greatly based on hardware.
(3) Time Machine and (ideally) Spotlight indexing should be disabled. Normally this doesn't make a huge difference but it can introduce variability. To disable TM, click on the icon in the menu bar, open TM preferences and clear the checkbox "Back Up Automatically". Remember to re-enable that when finished testing.
To disable Spotlight indexing, add each drive (including system drive) to the privacy tab of the Spotlight app in System Preferences. When finished testing remove those from the privacy tab. This will also rebuild all the indexes:
(4) Re-boot MacOS before starting a test sequence, and wait a few min before doing the test. I realize you may have already done this due to separate partitions.
(5) On newer machines where BruceX has a total run time of less than about 20 sec, it is better to append 5 BruceX timelines, do the timed test, then divide the result by 5. Even with all of the above procedures, there can be some run-to-run variation of a short test. Doing many individual short runs and taking the average is probably less accurate than doing one longer run of a 5x timeline. After appending the timeline 5x, do a SHIFT+Z to visually verify it is actually 5x not anything else.