It was
pointed out earlier that 200kbps seems to be the case.
I think you may be saying that there seem to be other throughput losses. I think you may be right, but that isn't the same thing as the hard cap that the Current line speed sets before those kick in.
It could be to do with the buffering involved in the prioritisation system. Given the overall throughput increase due to the increase in the number of customers, plus the radical change in the way people use the net in the last couple of years, it wouldn't be surprising if that side of the system wasn't creaking.
Queuing systems are complex beasties

. A small slowdown in one place can cause a huge jam elsewhere, as on motorways.
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 -
Plusnet UnLim Fibre (FTTC). Sync 57840/12740kbps @ 600m. -
BQM