You have to take into account 'VIRTUAL'.
EVERYHING in a VM is virtualized, the CPU, disk controller, NIC, and Memory. They are basically software emulations of the physical counterparts, so yeah it's going to be slow.
Slow is also relative. It's not an abosolute term, and it depends on your VM platform (CPU, Memory, Number of VM's, type of controller SAS/SATA, disk configuration, etc..).
Everything you do from within a VM is vritualized, you don't have direct access to the drives, so if you copy a file to a VM you are going over a virtualized NIC (turned into a switch) to a virtualized IO backplane, to a vritual layer of the disk, through virtual memory (depending on the VM, this could be too low, causing memory swap) ALL handled by a CPU that is virtualized by the VM Server, which oily gives a time slice of the overall CPU cycle.
So yeah it's going to be significantly slower, there are many variables, and any or ALL of them can affect traffic. Dedicated machines can control the stream better because they have exclusive access to controller and IO interrupts. VM's don't.
So data in that scenario may be slowed by the disk, or CPU inside the VM, we don't know.. but EVERYTHING should be considered, not just that NIC or the speed.
