You’ve probably done all the usual fault-finding stuff, e.g.
- checked with an independent speedchecker, like speedtest.net,
- unplug all the extension phones, plug the modem/router into the master socket – if poss the test socket inside the master,
- changed the ADSL filter from which you get your ADSL feed
- tested with a wired connector on one of your lappies - alone, with all else switched off. If that cures it, perhaps a neighbour has just started using the same (or near to it) channel as you are, or a cordless phone (or even doorbell – yes, honestly) is interfering.
If anything improves, then you have something to start on – else, keep hitting BT... for example, they may not have checked the noise on the line?
Good luck
J.