That's a good thing to check, but it doesn't work for a single stream of Long GOP material (the OP case). It can work for ProRes 422 on Max and Ultra machines.
This is a confusing area because of two different conceptually similar Compressor features. Compressor has long had the ability to run multiple parallel instances on a single well-equipped machine (Compressor>Advanced>"Enable additional Compressor instances."
If transcoding a ProRes file (say from ProRes 4444 to ProRes 422 LT), it could do segmented export whereby each Compressor instance would get a segment of the ProRes input file and produce a segment of the transcoded output file. Then Compressor (behind the scenes) would join all the files, presenting a single output. This had limitations because of the overhead of splitting and joining, so even on an M1/M2/M3 Ultra, which has 4 ProRes engines, it was faster but not breathtaking. For parallel transcoding of multiple ProRes files, the I/O bandwidth demands are high.
If submitting a batch of several jobs to Compressor, the multiple Compressor instances could handle those in parallel, even if not ProRes. On Ultra machines with 4 video engines plus 4 ProRes engines, this could help, assuming you had enough I/O, RAM, etc. But it was mainly useful for batch transcoding, not for exporting a single timeline faster.
More recently, "export segmentation" became available in FCP and Compressor, which (under some conditions) works on Long GOP formats such as H.264 and HEVC, and it works on a single timeline. Again, it's mainly useful for Max and Ultra machines.
In my testing of the OP case, I could see that FCP 12 was doing export segmentation for a 1080p 8-bit HEVC output, but Compressor 5.0 was not doing that, even though the "export segmentation" checkbox was set for both and the export characteristics were the same. But the performance difference when using export segmentation wasn't that great, even on FCP on my M1 Ultra machine. It was not enough to account for the 3x difference the OP mentioned, and there would be less benefit on a MAX, which has two video export engines, not four like the Ultra.
I did a brief test on Compressor 4.11.1, and it also didn't do export segmentation on the same 1080p 8-bit HEVC file that worked on FCP 12 and FCP 11.2. So there was no worsening of that on Compressor 5.0, at least in my tests.
But any Compressor preset can be used in FCP (as described previously), and in my tests, export segmentation worked on that, at least for 1080p 8-bit HEVC.
There are various built-in checks and constraints for H.264/HEVC export segmentation to work. The input file or timeline has to be above a certain length (which is not specifically documented), and there are various other internal checks done before it is enabled -- independent of the "export segmentation" checkbox.
E.g, Long GOP formats are classifed as having "Open GOP" or "Closed GOP" construction. If Closed GOP, each GOP is independent and can be processed in parallel. If Open GOP, they depend on each other so parallel processing is harder. There's no easy method like MediaInfo to determine which one a certain file is. FCP development is trying to auto-config and avoid a case where it actually slows down the export.
