Hi,
I know this isn't going to be particularly helpful, but...
I used to run a similar setup - MacMini quad core i7 with 16Gb driving 4x12Tb Seagate NAS drives in 5-bay thunderbolt enclosure where the 5th bay was a small SSD set up as an intent log. This was a fileserver for all-the-things (Time Machine, Plex, LMS, SMB, AFS, etc) all basically being served from one pool and various datasets therein running O3X 1.9.4. This has subsequently been upgraded but I'll talk about the old set up for reference.
Performance wise, this was never stellar, but I didn't expect it to be. Transferring to/from this setup would be able to saturate the 1Gbps network link for simple large-ish transfers. Throughput would wobble for many multi-Gb transfers in the 30-100Mb/sec range, but generally it was fine. So you should be able to get "acceptable" performance, but... things to consider or look at:
RAIDZ (with only 1 VDEV) - you're going to be limited (eventually) to about the maximum write performance of a single drive. I say "eventually" as ZFS will make extensive use of RAM to cache writes (and reads) prior to marshalling them and actually writting them out (making IO look a lot better than is physically is for a while), but when you hit the wall and have to rely on actually getting the bits to disk, performance can tank, hard. This might account for 20Gb data causing erratic write times (this can also be exacerbated by the size of the files).
Anyhoo... to the hardware.
The drives... those types of drives *may* be detuned for reliability and longevity (custom firmware). Dunno, maybe someone else can comment?
USB... is kinda arse for storage. Different people have vastly different experiences with ZFS on USB devices. I tend to only use ZFS on single USB disks but have been know to use it with striped USB enclosures and its been "ok". 4 or more drives I've always used a Thunderbolt enclosure (my 4x 12Tb drives are in an old Lacie 5-big TB1 box). Your Mac Mini might also lack UASP support.
The Mac Mini... RAM might be on the low side - the more data you throw at it in a short time will fill ARC fast and could cause stalls as ZFS has to scramble to actually push blocks to disk. The more concurrent activity you have going on (say loading new files, Plex transcoding, streaming, etc) the harder the disks will have to work and the more RAM will be required to smooth out that IO workload.
Genernally I find O3X performance to be ok for my needs (with the notable exception of when all the machines here are Time Machine backing up at the same time), but it certainly isn't stellar (the host now has 32Gb RAM and 10GbE and I can certainly push this host harder than the previous one). As @lundman might comment, for the Apple port, stability first and performance last in terms of priorities.
Oh and the benchmarks... always going to be a bit wonky due to how ZFS works.
Cheers!
James