RobertoS, I think I will do the domain thing. Thanks for the heads-up

You seem to have understood interleaving quite well.

In theory on ADSL Max (BT type connections that most ISPs use) interleaving has no effect on sync speed or download speed unless you sync near/at 8128 without it. In most cases of 8128 sync though not all, it will cost you 512kbps sync.

It always makes latency worse, increases ping times.

On LLU such as O2/Be it seems interleaving can actually increase sync speed. I don't know why.

In terms of the effect on throughput if interleaving was off, so data needed to be re-transmitted, I don't know of any commonly available info. See also what I said in the second paragraph. What is clear that some lines can clock up over a million a day with no lowering of speed test results. I expect without interleaving the retransmissions for those would slow the download. Yours are negligible.

I have seen my actual speeds fall when I got a few thousand per second! The router was spending all its time correcting them.