I came across many posts complaining about network speed on VirtualBox guests but none of the ideas discussed there made any difference in my case.
While fiddling with different settings in VirtualBox I also changed the type of emulated chipset from default PIIX3 to ICH9 et voilà – download speed was suddenly on par with the upload! With bridged networking, the speed on the guest is about the same as on the host. Speed on NAT networked hosts got bumped to about 200 Mbit/s (i.e. more than 10× boost).
I repeated the procedure, going from ICH9 to PIIX3 and vice versa with the same result – effect of the change on networking speed is immense.