Last update 11 Jan 2013
The new profiling system introduced during 2011 adjusts immediately to the connection speed, setting it to about 88.2% of that for ADSLx connections and 96.79% on FTTx connections.
This is a great improvement, as the apparent loss of speed is illusory, in that for ADSLx it covers most of the overheads associated with TCP/IP. (What the 3.21% loss on FTTx is for I don’t know, but at most fibre speeds does it matter?). The older version grouped speeds into wide bands, so that people near the bottom of the band were all right, but those near the top of a band lost all the speed between the bottom and their connection as well as getting the profile loss applied at the bottom.
If your throughput speed is consistently below the IP Profile shown by the BT Performance Tester then ask your ISP to investigate.