The high-level classes of the .NET framework for making web-requests hide that information from you (because for most applications it's irrelevant). You would have to use lower level classes like
Socket Class (System.Net.Sockets)[
^] instead in order to be able to calculate the transfer rate while the transfer is in progress. But socket-programming isn't a good starting topic for beginners..
If you want to give it a try anyway, here's further reading:
Socket Code Examples[
^]
The solution presented in this article might be interesting:
Monitoring network speed[
^]
Though it has the disadvantage that it will report the transfer rate of the "whole" network connection, so if there's another application downloading something it will be included in the reported transfer rate.