For what it is worth, the web-based Samknows speed test seems to consistently test well below line rate here (https://speedtest.samknows.com/)
Another day, another speedtest. I looked, this one starts 8 downloads from your browser, to same server IP 80.71.21.229, measures the total amount of data downloaded over 12 seconds, and ESTIMATES the speed. I know I keep banging on about this, but take any speedtest website with a pinch of salt, its just an estimate guide of your speed. *IF* they could accurately measure the speed they would all give the same result.
A far more accurate way to confirm what you speed is is to look at the interface on your firewall router when you are loading the network, using multiple devices on your LAN. I honestly believe you are chasing your tail trying to get the results you want from a speed test website with a fast FTTP connection.
I would also suspect that speedtest traffic from websites is given a low priority on the various networks. It makes sense ISPs would not want speedtest traffic run by a very few people to impact the many majority that are streaming music/video and making VoIP calls.
I think @Alex as sort of confirmed this:
Just wanted to confirm that despite single-threaded speedtest.net results off Zen server still maxing out at 300Mbit I was able to download in Firefox at 100MB/s earlier which would rather confirm everything is fine under real-world scenarios.
I have personally found some gateways on Zen perform better than others, find one that works for you, and stick with it. If PPPoE drops, redial until you get back on the one that performs best for you.