I personally have never known that changing the MTU setting can speed up your connection, as the MTU is the maximum transmition units that is sent to your pc from each web site and if the MTU setting is incorrect then some web sites will not load on your explorer page, the speed at which you download the page will not make any difference.
If you are supposed to get 20MB speed then you must be on cable and not ADSL as ADSL is a max of 8MB, so i would ring up Virgin or whoever and check the speed you should really be getting.
See these details posted on the netgear site, and you'll notice there is no mention of speed:-
Details About MTU
A packet sent to a device larger than its MTU is broken into pieces. Ideally, MTU would be set to the same — large — value on all your computers, routers and switches, as well as on all the parts of the Internet that you access. But you cannot control the MTU on the Internet, and in practice the optimum MTU size on your LAN is related to your hardware, software, wireless interference, etc.
- Tweaking MTU size may work well in one situation, but cause performance and connection problems in others.
- When network devices with different MTU settings communicate, packets are fragmented to accommodate the one with the smallest MTU.
- Windows XP sets MTU automatically, that is, it optimizes computer MTU for you. This Microsoft article explains resolving lack of connection to a broadband ISP using Windows XP: How-To Configure Broadband Connections Using PPPoE.
- Once a network device fragments a packet, the data stays fragmented until arriving at the destination computer.
Setting MTU size is a process of trial-and-error: start with the maximum value of 1500, then reduce the size until the problem goes away. Using one of these values is likely to solve problems caused by MTU size:
- 1500. The largest Ethernet packet size; it is also the default value. This is the typical setting for non-PPPoE, non-VPN connections. The default value for NETGEAR routers, adapters and switches.
- 1492. The size PPPoE prefers.
- 1472. Maximum size to use for pinging. (Bigger packets are fragmented.)
- 1468. The size DHCP prefers.
- 1460. Usable by AOL if you don't have large email attachments, etc.
- 1430. The size VPN and PPTP prefer.
- 1400. Maximum size for AOL DSL.
- 576. Typical value to connect to dial-up ISPs.