Recently I had the need to use several different DOS VMs that all used a SMB network client. Although I did not use networking heavily, I noticed that there are massive differences in performance between the VMs. Copying a circa 42 MB file would take anywhere between about 5 seconds and 49 minutes (not a typo). What’s more, some VMs were fast in both directions, while others were very slow sending and yet others were very slow receiving. Since in all cases the VMs communicated with the same server (Synology NAS running Samba) from the same host (AMD Ryzen 3800X running Windows 10 and VirtualBox), there really should not be that much performance variation, and yet there it was.