People are seeing very poor performance with these SMR drives and Synology as well, even in normal operation. A great example is http://blog.robiii.nl/2020/04/wd-red-nas-drives-use-smr-and-im-not.html

Join the Inner Circle? The Inner Circle is a community of like-minded individuals who are passionate about the same things you are. You’ll have the opportunity to ask questions and get replies from me and other Inner Circle members who are dedicated to helping each other out. I bought 2 WD40EFRX-68WT0N0 3 years ago. 1 failed after 2,5 months. RMAed. Got another WD40EFRX-68WT0N0. The RMAed one lived nearly 2 years and failed again. RMAed again and this time I got WD40EFRX-68N32N0 - purportedly the newer one. This time I bought additional warranty so I should have 5 years of peace if it fails.Robert – I generally look for low-cost CMR drives, and expect that they will fail on me. While I expect the drive failures, I also look for a predictable level of performance during operation and rebuilds. In both cases, the WD Red SMR drives would not work for me personally. I will also say that a likely part of the problem here is that these are DM-SMR drives that hide the fact they are SMR from the host. SMR drive support is getting better when hosts know they are using SMR drives. Western Digital’s exclusive NASware™ technology fine tunes drive parameters to match NAS system workloads which helps increase performance and reliability. Would be worthwhile to at least update the following articles with a warning to avoid SMR HDDs when using ZFS:

NAS drives are always a gamble, SMR or not, you should always keep away from the cheap HDD drives and that also includes cheap SSD’s if you are trying to have a NAS that have a good performance in a Raid setup. Few of them are SMR and other are CMR drives. You can identify them here https://nascompares.com/answer/list-of-wd-cmr-and-smr-hard-drives-hdd/Unfortunately, while the SMR WD Red performed respectably in the previous benchmarks, the RAIDZ resilver test proved to be another matter entirely. While all three CMR drives comfortably completed the resilver in under 17 hours, the SMR drive took nearly 230 hours to perform an identical task. WD40EFAX FreeNAS Resilver Great article as always. 🙂 Great to see some hard facts related to this after reading about it from others. The RAID rebuild problem you refer to is a peculiarity of ZFS. It does not occur in Hardware Based RAID and it does not occur in Windows RAID or Storage Spaces. It does not occur in other Software Defined Storage solutions.