Just had FTTP installed - full 300/50 Mbps and as far as I can tell everything is running fine apart from one website I use for work. It runs really slowly, including any downloads I select. I have to sign in to the website but there's no special authentication required. I have an iPhone app version of the website and it has the same issue.
If I switch to tether with my phone it runs fine, as does running Tunnelbear VPN to the UK (obviously I'm in the UK too) - in which case I get downloads at 50 Mbps+
Without Tunnelbear pages take ages to load and downloads are at 10 Kbps.
Pinging gives lots of this kind of thing when I'm direct via BT:
Request timeout for icmp_seq 26 Request timeout for icmp_seq 27 Request timeout for icmp_seq 28
and it gets more regular the longer it pings. Via VPN I don't get any timeouts.
I've tried changing DNS from Google, to 188.8.131.52 to OpenDNS but nothing changes this.