AlphaVM currently emulates 100 and 10 mbit NICs. However, it does not limit the network throughput, because the emulated NIC actually operates at the speed of the underlying host NIC, no matter what the speed of the emulated hardware is. On a 1gbit NIC the actual speed will be 1gbit. Only the nominal speed will be slower.
Please note that, if you use AlphaVM-Basic, your bottleneck will be the slow CPU. It will limit your actual speed.