Jumbo Frames currently are only relevant to LAN data transfer, they are not used on the Internet as a whole. In fact the router would have to fragment those frames before transmitting them to your ISP, so potentially it could actually make performance worse.
They were primarily introduced for Gigabit ethernet as the normal frame size is perfectly sufficient for maxing out 100Mbit ethernet. In fact, if you look at the linked article above again you will notice it shows the speed BEFORE Jumbo Frames was 46MB/s. That is about four times the speed Infinity/VDSL2 could do if BT had no speed limits on it, around ten times the speed of Infinity today. So clearly, Jumbo Frames are not the answer
On the other hand there are a lot of TCP/IP parameters that can affect broadband speed, but if you are using a modern operating system on your computer that is highly unlikely to be the problem as they automatically optimise themselves for faster speeds.
Its perfectly normal for speed tests to not show your full speed. On a 40/10 connection like Infinity the average speed is more like 37/8 but can vary. So it sounds to me like your connection is working perfectly normally.
Edited by alexatkin (Wed 11-Apr-12 03:18:36)