so who is your isp?
speedtest.net is not running great on my phone either, that tells me I have an issue somewhere with the phone's tunable, or its wifi connection or the speedtest app, it doesnt mean the test itself is broken. Might also be an issue with my isp at the time of the test or the speedtest server or the transit link. With that said I rarely run speedtest.net on my PC now as I much prefer tests that use less threads. like speed.of.me and tbb, also dslreports.
I'm with Zen and have been on at least two different variations of their network, I was also on Origin Broadband both on BT backhaul and before that on Digital Region before it closed down. I have always had varied results from those speed tests throughout.
I know sometimes single-thread speeds can be hit by contention but its strange how their multi-thread can do too, with only dslreports being fairly consistent. Pretty sure its not the router either as I have been through various over the years and as I have been running pfSense on an Atom for some years now there is plenty of spare overhead, plus real-world performance is never a problem. We regularly have three of us watching Netflix at the same time with one of them in 4K without any issues.
Anyway the point is, an artificial speed test can only be a guideline. You have to monitor real-world performance as well to get a clear picture. The BT/Openreach speed tests in particular can also be especially pessimistic.