Just thought I should post this as it took some time to figure out.
2800VG, connected to Plusnet via BT line, 2.7Km from exchange by road, pretty good line stats (13-16dB SNR margin, steady 40dB Attenuation). Up-to-8Mb service and no throttling from the ISP.
Sync speeds to the exchange were steady at 4 to 5Mbps but I'd noticed by broadband performance had been getting worse, although I'd just put up with it. Eventually I decided to look into things a little more, partly because I had MRTG logging everything in the background and it was clear from this that there was no issue with the line, but speed was definately not what it was a year ago.
Despite the line stats looking good, I double-checked things by moving the router into the master port, turning the whole house off, doing the bell wire fix, yada yada yada - but line stats stayed the same, the sync speed stayed the same, but my effective throughput using the BT SpeedTester was only 611kbps despite being on a 3.5mbps profile.
I fiddled with the connection settings, trying different GMT/ADSL profiles (I'm only on an ADSLMax line here; no ADSL2+). But no joy.
It was with the help of a neighbour who had the same modem and ISP, but was seeing 3.5 meg link, that I started to suspect the router.
I wiped it back to factory default settings, did the minimal reconfig to get it connected, and presto - 3mbps link straight away.
At a loss to explain why. Maybe this was introduced a while ago by a minor configuration change or fireware update; it's possible. I guess the lesson learnt here is to check your performance after any updates to your router config, although I'm not entirely sure this is what happened in this case. Maybe an exchange-level change can conflict with an existing setting in the router? Don't know; I'm not technical enough.
Suffice to say if you suddenly see your real-world performance drop to ~600kbps despite good line stats and sync speed, suspect your router and try a basic reset to factory default config (backing up your existing config, naturally).
