Windows 11 has had a bug for a long time that slows down the write speed of hard drives and SSDs. Microsoft is releasing a fix for the problem under patch  KB5007262. 

Recent tests by Reddit user PleasedPen25317 showed that the random write speeds of the Samsung 980 Pro SSD went down a lot when Windows 11 was installed on any partition that had the SSD. It must have been more widespread than previously thought because the problem affects all drives, not just NVMe SSDs, which are very, very fast.

November 22, 2021—KB5007262 (OS Build 22000.348) Preview - 

Addresses an issue that affects the performance of all disks (NVMe, SSD, hardisk) on Windows 11 by performing unnecessary actions each time a write operation occurs. This issue occurs only when the NTFS USN journal is enabled. Note, the USN journal is always enabled on the C: disk.

Read more about it here, a whole lot more has or will be fixed.

Microsoft acknowledges slow NVMe SSD write speeds in Windows 11 - working on a fix.