Maximum throughput on ADSL2+ is about 0.84 x connection speed. So a 16% loss. Nearly all that is TCP/IP overheads.
84% of 16Mbps is 13,440kbps. Your loss of speed looks to be due to a lower connection speed than when with TalkTalk.
Can you see your attenuation and noise margins in the router stats?
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - AAISP Home::1 80/20. Sync 59997/15142kbps @ 600m. - BQM