RAID 5 very bad performance

I bought new PC and 6 new drives of Western Digital RED 6TB and combined them into RAID 5.

3 Disks WD60EFAX-68SHWN0 and 3 Disks WD60EFAX-68JH4N0

Initialization was running about 1 week. And after it, when its complete I checked performance of Virtual Disk of this drives and performance was very very bad. Read about 20 MBps and Write between 10 to 0.5 Write (Sequantial).

This was looks like quite strange for me and I contact with motherboard vendor to solve this issue. After a log journey with them (drivers, firmware and chipset updates) I decide to broke this RAID and tested each disk separately. Performance was really good, about 200 MBps to 180 MBps for Read and Write (In Sequential).
Then I created RAID 0 and performance was really great. I start thinking maybe this is problem of model type. I created RAID 5 from all WD60EFAX-68SHWN0 disks and they gave same bad performance. I did same with second model and same.

Then I start thinking, maybe my motherboard has some issue with this disks or something with RAID 5.
I took this disks to my NAS WD Ex4100 took all disks from it. There was 4 disks or Wester Digital 1 TB Blue disks (WD10EZEX) which was gives good performance and swapped between them.

Now in my PC has 4 disks of WD Blue 1TB in RAID 5 which works really awesome and NAS with 4 drives (2 disks from model 1 and 2 other from model 2) in RAID 5. And now performance are bad on NAS.
Initialization took about 3~4 days. Read about 10~5 MBps and write hardly reaches 1~2 MBps.

What should I do?
Is there some compatibility issue or this is disk problem?
Or this disks are not design for RAID 5?
I took this disks to shop and asked them to check, but they told me that they will check each disk separately.

  1. WD60EFAX = SMR = not suitable for any RAID, except RAID0 (pure stripe write) or except JBOD (just for sequential I/O)
  2. you can use it in mentioned disk group setup or as simple disk for a backup purposes. Never in RAID.
    SMR is useful technology for exactly defined usage. Otherwise you can see your own evaluation.
  3. you can purchase WD60EFRX, it will solve your problem.

So you say Western Digital sales disks that are designed exactly for NAS but not compatible for any type of RAID except RAID 0 (striping) without providing any information about it to customers?

SMR Pros/Cons are well known from 2015, when was first disk drive available commercially.
But from April 2020 is know heavy panic about it in world wide range.
When your shop knows, that your purchase was driven by RAID building and no one stopped you from their side, then you have got a good reason to ask them for a replacement. Only when you have an evidence.