If a filter is not doing its job properly then some of the ADSL signal reaches the telephone equipment. This may or may not result in audible interference on the phone equipment but also results in a distorted version of the ADSL signal being fed back onto the line by the telephone equipment that is not designed to handle ADSL frequencies. This then increases the noise, lowering the SNR margin, generally causing a slower connection. This is why quite often just changing a filter will result in a speed increase.