You forget my point about your post
You do not know that the (agreed) loss of throughput is due to the Current Line Speed system.
I didn't forget, but I equally didn't give enough emphasis to what my experimental data shows.
You're right that I don't *know for certain* that the problem can be put down to the CLS system, but over 4 years, the various results I have seen tell me that this is the only location it could be. If I were part of Plusnet, and had access to their internal systems, that is the only place I would be looking ... but I'm not, so I can't investigate further, and have to leave it at that. I accept that.
Why am I convinced that the cause lies within the CLS system?
I started with no correlation. The very earliest issues appeared to be related to switching gateways on PPP-reconnection, but there was no strict correlation to that either (much like the recent congestion issues).
Over time, I had eventually made an informal correlation between the highest speeds & an excessive CLS-value, and the same correlation between the lowest speeds and "normal" CLS-values. But I had no way to change CLS-value in a controlled manner, and perform decent experiments; but occasional changes (either at manual request, to set artificially high values, or automatically-induced, to set regular values) helped build the case over the course of a couple of years.
The final correlation came when, as I mentioned in my original post, I did some testing with Dave Tomlinson. He made changes to the CLS value that helped establish, in my mind, the direct correlation between CLS value and truncated speed.
(Aside: As things ended, I had to stop detailed testing because of health issues. The next thing planned would have investigated how CLS settings affected behaviour under load - giving more indication of QoS balance. I wasn't up to concentrating on the detail needed for that, at the time)
For me, I am happy to continue to believe that the loss of throughput is an established feature of PN's connections, and I am as happy as I can be (without becoming a PN employee) that the CLS system is the host of the cause. The combination of these two is what makes me say that "The question of whether this happens has been put to bed."
I further state that "The question about why it happens has not" because we have had no input from PN staff; we cannot know what it is about the CLS system that turns the loss of throughput into fact. Putting the blame on CLS is not the same as understanding why CLS behaves this way.
To me, both statements stand, and are not a contradiction.
I do recognise that, for anyone else to believe my position, they also have to accept my interpretation of my experiments & results.
And with that, I'll have to withdraw from the debate, as I won't be around much over the next two weeks.