Someone on Reddit sent me to http://netalyzr.icsi.berkeley.edu/
This reported the following:
Network buffer measurements (?): Uplink 2500 ms
, Downlink 919 ms –
We estimate your uplink as having 2500 ms of buffering. This is quite high, and you may experience substantial disruption to your network performance when performing interactive tasks such as web-surfing while simultaneously conducting large uploads. With such a buffer, real-time applications such as games or audio chat can work quite poorly when conducting large uploads at the same time.
He suggested this is likely the modem being poor, or a bad line.
One of my original theories was a huge buffer somewhere (probably the modem); think I'll try and get hold of another one and test that.
I'd be interested to know what values other people get from that test.