To determine if the issue is the line, go into the router's diagnostics webpages and look at the Noise Margin, or SNR (signal to noise ratio) figures. To get a decent data rate, they should be around 6dB or better for downstream and upstream. If the figure is good, then the slowness of the connection could be down to contention with other users, or to the site(s) you're visiting limiting the bandwidth. Premium download sites (i.e. ones like Netflix, that a subscription is paid for) tend to have higher network bandwidth to their sites. Bandwidth tests tend to go to dedicated servers to get the maximum data rate possible, and possibly even alter the contention ratio for the duration of the test, to make it look more rosy. A bit like VW's "test mode".