Lolly, if you have OSX, and want to change the MTU size, it's easy.
First, go to speedtest.net and run a speed test and note the results. You might want to run it three times so you can average.
1. Open the System Preferences under the Apple menu.
2. Open the "Networking" item from the System Preferences Panel.
3. Assuming your DSL connects to the Ethernet, you should see in the left-hand column, "Ethernet" with "Connected" under it and a green spot left of that. Click it if its not already hilighted.
4. Now at the bottom-right of the panel is a button labeled "Advanced..." Click it.
5. A drop-down panel will appear. Click the "Ethernet" tab.
6. Change the "Configure" item from Automatic to "Manually"
7. Change the MTU item from "Standard (1500)" to "Custom"
8. Enter 1400 in the box below that item.
9. Click OK. The drop down panel should appear.
10. Click "Apply"
Now, you can see if it changed anything by again going to speedtest.net and running the test.