SSH / SCP / SFTP transfers extremely slow when using bridged VLAN

[nirgants]

Setup

• Host is Devuan Ascii 2.1 (Debian 9 ~stretch), VirtualBox 6.1.2r135662
• Guest is Devuan Ascii 2.1 (Debian 9 ~stretch)

Using NIC type virtio:

• scp uploads to the guest bridged with vlan use full speed of connection (1gbit/s LAN)
• scp downloads from guest bridged with vlan are as slow as 40KB/s and never higher than 170KB/s

Using any other NIC type:

short: Just the other way round

• scp uploads to the guest are extremely slow (when uploading from a Linux machine) or even impossible (when uploading from a Windows machine: just hangs after initial 3 Kbyte).
• scp downloads from the guest bridged with vlan use full speed of connection (1gbit/s LAN)

Checks

• scp transfers from or to host are full speed of connection (1gbit/s LAN)
• scp transfers from or to guest bridged not vlan use full speed of connection

Moving the guest to a different host, Debian 8 (jessie) with VirtualBox 4.3.36_Debianr105129:

• scp uploads to the guest bridged with vlan use full speed of connection (1gbit/s LAN)
• scp downloads from the guest bridged with vlan use full speed of connection (1gbit/s LAN)

Additional Information

 https://forums.virtualbox.org/viewtopic.php?f=7&t=96066&sid=5be93bccf4f9e91bac3d740333cbba1b

[ADV]

I'm having exactly the same problem. I've not tested what happens without VLANs yet, but as Nirgant says changing virtio driver with intel's reverses the problem. I'm using Virtualbox 6.1.14 with kernel 4.16.11-100.fc26.x86_64 fot host and my virtual machine is 4.18.19-100.fc27.x86_64.

[ADV]

I finally solved it. As the "Networking recomendations" say, offloading should be disabled on the host nics. I just used "ethtool -k" to disable it and the problem dissapeared!