sudden drop in speed of /dev/net/virtio under VirtualBox (correlated with "virtio_net: ioctl: unknown message 22dd" in serial log?)

[taos]

VirtualBox: 5.2.18
Host system: archlinux (8 GB RAM, 4 "cores")
Guest: Haiku 64bit (2 GB RAM, 2 CPUs, NAT with virtio-net), at the moment at hrev52302

Quite often (most of the time, already after booting to the desktop) the speed of the network connection drops suddenly (from ~ 4 MiB/s to below 1 KiB/s).

If this happens, the network speed is so slow that even SoftwareUpdater can't always validate the checksums of the repositories but times out. WebPositive can't show the Haiku homepage. Vision can't connect to freenode.

Around the same time, the following messages start to appear in syslog and serial log:

[33mvirtio_net:[0m ioctl: unknown message 22dd

Followed by hundreds of:

Last message repeated 16 times.

If I'm especially lucky, the speed increases again after a while to up to 6 KiB/s and I can try to clone haikuports from github which then often ends after 30 min with

error: RPC failed; curl 18 transfer closed with outstanding read data remaining
fatal: the remote end hung up unexpectedly
fatal: early EOF
fatal: index-pack failed