WD RED WD60EFAX performance degradetion caused by SMR technology

Goodmorning everyone,

I am experiencing substantial degradation of read and write speed on my QNAP NAS.
The NAS is equipped with 4 WD RED 6GB (WD60EFAX) purchased in May 2019.

For these hard drives I have read that SMR technology is used.
From what I read hard drives with this technology should not be used on NAS.
I also see that last year a class action was taken against Western Digital in Canada precisely for the use of SMR technology instead of CMR, which should be the only one used on NAS.

So I wonder, since these 4 hard drives are purchased and used in Italy, is there a way to access a replacement?

Thank you in advance,
best regards