Well I have a BT Infinity connection that is a theoretical maximum of 40Mbps download and 2.0Mbps upload (limitation for the account I have).
Given that this is a Fibre To The Cabinet, fibre optic cable to the local street cabinet and then copper cable for that last part (telephone connection) to the home. So some degradation happens for the copper cable from that theoretical maximum, plus the usual degradation as the number of hops in the connection.
I use speedtest.net to periodically check my speeds and in all of the time I have had broadband (only 15 months) I have found it close to my theoretical maximum. These tests run with and without the web shield enabled and in all honesty I find little difference between the two, in fact at times the connection/download speed is faster with the web shield enabled.
I have run a quick test with the web shield enabled and on three different servers and broadly the results are the same (see image), why two of them don't report the upload speed but they too have always be around 1.6-7Mbps.