can you just post your firmware version, just to see which one you have, maybe you don't have none of those two and you have version 1 which is also good, install Mac OS on another drive then format Samsung 960 as Mac OS journal and run a benchmark a few times, Ajax or black magic speed test, then reformat the drive as APFS then run the benchmark again, it is weird. I had 960 and they never gave me a problem with APFS, are you sure you are using the same APFS.efi that comes with the Mac OS system that you are using, what I'm trying to say is you are not using APFS.efi a few version behind in a newer os, if you are using a patched version, try with a none patched APFS.efi maybe the one that is patched is causing you the problem, but if you have the bad firmware then that is the problem, because that is the symptoms of the bad firmware, it throttle all the way down to a crawl
About This Mac - System Report - NVMExpress -
look at revision ??????
what is the number there
ok you see you mentioned that you updated the firmware to the latest, I just check Samsung web site and the latest firmware is the bad one, I really don't understand why Samsung hasn't pull that bad firmware, for what I know maybe they update the nvme driver to work with that firmware but that is windows driver only, it will not work with Mac OS, the other only possibility is that you can update to a higher firmware that is available at Samsung web site if you their software something magician I just don't remember the name right now, maybe they don't have the new firmware available for download but you can update to the latest firmware if you use Samsung magician software, if you have the bad firmware there is no way around, Mac OS journal, APFS, ex fat, NTFS what ever you choose it will be the same story all over again, unless there is a new firmware with the fix, bad firmware sysptom after a minute the drive will go down to 300 mbs, the problem is not APFS unless you are using an old version in a newer system or whatever the reason might be, please post a screenshoot of your firmware