Disk transfer speed deterioration since installing Windows 11

I am not sure whether this question belongs in Windows 11 or PC hardware, it seems to be hardware but the change in O/S has introduced the issue. If an admin wants to move this into a more appropriate forum please feel free to do so.

Amongst other devices on my home network I have a home-built desktop PC with multiple disk drives and a Synology NAS. I recently gave the PC a heart and brain transplant but retained all my disk drives, and at the same time replaced Windows 10 with Windows 11 Pro.

Since then although everything – Windows and apps – seems to be functioning fine, I have noticed two odd things relating to disk transfer speeds.

Both versions of the PC had an SSD drive as the primary C drive, and the following 3 SATA  drives:

• D: Seagate Constellation ES.3 3TB (7200 rpm)
• E: Western Digital WD Black 6TB (7200 rpm)
• G: Western Digital WD Red 6TB (5400 rpm)

The old PC could copy data from any of these disks to the NAS at 110MB/s. That’s what I would expect on a gigabit network, and it’s what I got. But the new PC can only copy data to the NAS at 55MB/s.

So far as I know nothing has changed in the system other than the version of Windows and the motherboard (was an ASUS Z-390 ROG-STRIX, now an ASUS Z-890F ROG-STRIX). But for some reason the data transfer across the network has halved – just from this PC, not from other machines.

I am sure the problem has nothing to do with the NAS or the network, which have not been altered in any way, nor the disk drives themselves, which have not been changed – the rebuild only replaced the contents of the (SSD) C drive. But there is something fishy about the HDD performance, because this brings me to the second observation.

I never bothered noting any transfer speeds between the disks in the Win 10 incarnation, because all copying just ran at a more or less constant rate and I never had any concerns about performance. But in the Win 11 version I have been seeing very erratic behaviours, slow downs and pauses.

As a test I created a data file of 5GB and copied it between all the disks. I renamed the file each time to try to avoid caching (although I don’t know for sure that this would prevent it, or if it would be a problem if I hadn’t). My experience tells me that the rate and elapsed time of all of these transfers would have been consistent in the Win 10 environment.