While not the same issue, I took four 500GB Samsung 850 EVO drives and created a Storage Space out of them for Hyper-V VMs. Under any sort of load the volume would reach ~1 second latency. This was on a SAS controller in JBOD mode.

Switched to some Intel 480GB DC drives and performance was in the low milliseconds as I would have thought any drive should be.

Not sure if I was hitting the DRAM limit of the Samsungs or what, spent a bit of time t-shooting but this was a home lab and used Intel DCs were cheap on eBay. Granted, the Samsung EVOs weren't targeted to that type of work.

850 EVO is basically the lowest tier consumer device, from what I have read those devices can only handle short bursts of IOs and do not perform well under sustained load.

The Samsung 850 EVO drives probably used an SLC write cache. A small portion of the NAND is configured to use as an SLC write buffer so they can handle a burst of writes faster and later move them the the MLC/TLC region. This is sufficient for typical consumer workloads.

Another thing you will notice is the 850 EVO is 500GB capacity while the Intel one is 480GB. The difference is capacity is put towards overprovisioning which reduces write amplification. The idea is if you have sufficient free space available, whole NAND blocks will naturally get invalidated before you run out of free blocks.

Could be garbage collection pauses. You could try wiping them again with zeros or doing a drive specific reset and see if the performance is normative.