Asymmetric SMB performance: fast VM → client, very slow client → VM, despite near-gigabit network (Windows 11 ↔ Windows Server 2019 VM)

[African-devision]

Hello,
I’m facing an issue with asymmetric SMB performance between two Windows systems, while raw network tests show almost full 1 Gbit/s throughput.
Environment

1. Hypervisor
   • Proxmox VE host.
   • A Windows Server 2019 VM running on Proxmox, hosting an SMB share (e.g. \\192.168.xxx.20\shared\iso).
   • VM NIC model: VirtIO (paravirtualized), attached to bridge vmbr0.
2. Client PC
   • Windows 11 laptop (physical machine).
   • Connected via wired 1 Gbit/s Ethernet.
3. Network
   • Standard wired 1 Gbit/s LAN, no VPN in between.
   • Switch ports and NICs report 1.0 Gbps Full Duplex.

Symptoms

• VM → Client (SMB copy from VM share to laptop):
  • Copy speed is around 100–110 MB/s (Explorer shows e.g. Speed: 108 MB/s).
  • This matches what I expect from a 1 Gbit/s link.
• Client → VM (SMB copy from laptop to the same share):
  Copy speed drops to about 13–25 MB/s.
  Example robocopy result:
  
  Bytes : 5.501 g 5.501 g
Times : 0:16:03 0:04:00
Speed : 24 511 513 Bytes/sec. ≈ 23.4 MB/s
  
  Explorer shows similar speeds (around 13–25 MB/s) when copying from the laptop to the VM share.
  So SMB is fast in one direction and slow in the opposite direction.

What I have already tested​

1. Raw network tests (iperf3)
   • Between VM and laptop:
     Between VM and Proxmox host: iperf3 reports tens of Gbit/s (as expected for in-memory virtio traffic).
     [SUM] 0.00-30.00 sec 3.10 GBytes 887 Mbits/sec sender
[SUM] 0.00-30.00 sec 3.10 GBytes 887 Mbits/sec receiver
     → Almost 0.9 Gbit/s in both directions.
   • This strongly suggests that the network path itself is not the bottleneck.
     
2. SMB settings
   
   On the server side (VM / Server 2019):
   Get-SmbServerConfiguration | fl EncryptData,EnableSecuritySignature
returns:
EncryptData : False
EnableSecuritySignature : False
   On the client side, SMB client signing is not required.
   SMB encryption and signing are disabled to rule out their overhead.
   
   
   
3. Antivirus / Defender
   • Real-time protection was temporarily disabled on both the client and the VM during tests.
   • No significant change; asymmetric behavior persists.
4. Disk performance
   
   • The share on the VM is on drive D:. Inside the VM, winsat disk for D: reports very high sequential performance (virtio SCSI with cache):
     
     
     Disk Sequential 64.0 Write ≈ 1189 MB/s
   • VM disks are attached via VirtIO SCSI (VirtIO SCSI single controller).
   • The fact that VM → client SMB copy reaches 100+ MB/s confirms that reading from the VM disk and writing to the client disk is not an issue.
5. Ports / firewall
   
   • SMB works in both directions via TCP 445.
   • Since Explorer and robocopy can access the share, ports are clearly open.

Summary of the issue​

• iperf3 between VM and laptop: ~0.8–0.9 Gbit/s both ways
• SMB copy VM → laptop: ~100–110 MB/s
• SMB copy laptop → VM: ~13–25 MB/s

So the problem appears only when uploading from the Windows 11 client to the Server 2019 VM via SMB, while raw network bandwidth and disk performance are sufficient.

Questions to support​

1. What could cause such one-directional SMB slowdown (client → VM) when:
   • iperf3 shows near-gigabit throughput in both directions;
   • disks on both sides are fast enough;
   • SMB encryption and signing are disabled;
   • antivirus real-time protection has been ruled out?
2. What additional diagnostics do you recommend to pinpoint the issue?
   • SMB client/server tracing (ETL logs)?
   • netsh trace or specific PowerShell commands (e.g. New-SmbClientSession, Get-SmbSession, etc.)?
     If possible, please provide a recommended set of commands and which log files you would like to see.
3. Are there any known issues or hotfixes involving SMB performance between Windows 11 and Windows Server 2019 that could lead to asymmetric transfer speeds like this?

Thank you in advance for your help.

Best regards