I am running about a dozen various Linux guests under Windows 10 VirtualBox 5.1. On a guest, I have noticed that while upload speed is satisfactory (ca. 300 Mbit/s in bridged mode, that’s about the speed achievable on the host), download speed is terrible (less than 2 Mbit/s).
Guests are configured with NAT networking with network interfaces being virtio-net. There also is one guest configured in bridged mode I am referring to in the previous paragraph.
Host CPU is Core i5 4460. Linuxes are stock Debian 7, 8, 9, Ubuntu 12.04, CentOS 7, 8, both 32-bit and 64-bit editions. Bridged guest is Funtoo Linux with a custom kernel.
I have changed virtio-net to emulated Intel PRO/1000 MT Desktop with no measurable difference.