Look at the route your line takes from entering the house to the master socket. Are then any electrical devices, fridge, TV, boiler, HIFi &c close to the cable?
Moving anything electrically noisy away from the line might get you a little extra noise marging and even 0.5 dB will give you a speed improvement.
Which modem are you using? There are some, such as the 2wire2700s which are good on long lines and again changing modem/router might gain an extra 0.5 dB which will translate to a speed improvement.
M H C
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