Very bad performance within Windows on Proxmox (with Virtio)

[Agent Purple]

Hi,
I am using Veracrypt since a long time. Thank you for that awesome tool!

Recently I moved my Windows Server (2012 R2) to a newly setup Proxmox machine.
The hard drives (HDD storage drives) are passed through via Virtio. It gives a good native performance if I format the drives unencrypted in OS.
As my drives are encrypted with Verycrypt since "ever" (for reasons) I just mounted existing drives on the new system.
Technically everything is fine and read performance is as expected.
But write speed is bad, not to say fucking bad. It drops from native speed to just ~20-25 MB/s.
I use the portable version (seems to be only 32-bit), because the installer version has the side effect that bare metal Windows backup does not work anymore. It seems that it has side effects on Volume Shadow Copy.

The performance is the same not depending if I create a container or parition encryption. I tested a lot with it, but could not find a solution.

Can you help me with this issue?

See also https://forum.proxmox.com/threads/performance-for-passthrough-discs.98929/

[BBB]

Hi guys,
I know it's an older thread, but I do confirm that that issues in combination with Proxmox still exists, resulting in writing speeds at around 30MB/s throughout (tested on SSDs and Windows Server 2022)

Things I tried:
- several Proxmox versions from old 7.x branch to the latest 8.1.4 and its respective virtio-drivers.
- different HW
- using container instead of full disk (both on the system disk and on the passed through drive)
- passing through the drive with different Bus/Device types (virtio, sata, scsi)
- playing around with different mounting types in VeraCrypt
- VeraCrypt installed vs. portable

The HW(s) mentioned work(ed) perfectly fine in conjunction with ESXi

Since Proxmox will gain importance with the cancellation of ESXI, I'd appreciate if someone could have a look. (@idrassi, maybe?)

Thanks in advance