No the BT network is not that congested, there is plenty of spare capacity. That does not mean to say that congestion never happens, sometimes capacity needs increasing in a certain area. Quite often a quick fix but sometimes requiring extra infrastructure at the exchange.
First which exchange area are youin?
Second can you run a speed test HERE, once the first test completes click on Further Diagnostics and post the result of that test. Ignore the reboot hub.
it would be good if you had the link as have not seen that one
It actually shows it shared between 128 users on that diagram, 4 ports on each splitter.
I doubt every port would be running flat out simultaneously at any point though.