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There's still no point in gigabit broadband
(shkspr.mobi)
This is a most excellent place for technology news and articles.
I have made the measurements, and at 500 Mbit/s I actually got a bit more than 5x what I had at 100 Mbit/s. Actually my 500 Mbit connection ran as 550, because the rated speed here is the guaranteed speed of the connection. So the only limitation is the server at the other end.
It is true however that 1 Gbit/s didn't quite double the 500 Mbit/s speed, Actual measured facts beat speculation.
But your examples of steeply diminishing returns are not true.