Here's an update.
With the help of another Lightning Fibre customer, a business customer, I've been able to confirm that the low single thread download speed issue isn't isolated to me. It's a somewhat strange behaviour in my opinion because it seems to start off near gigabit generally and then a few seconds later drop down to 200-400 megabits as if there's some kind of burst shaping going on, but that's probably not the case and just looks like it.
Me and another Lightning Fibre customer performed a local test, pretty much, using iperf3 between our Lightning Fibre IP addresses. The results showed near gigabit speed in either direction.
They also did a speed test on speedtest.net (single thread) using the Lightning Fibre server and they unfortunately had a similar result to me. They also did a speed test from their dedicated server in London using the Lightning Fibre server and were able to achieve multiple gigabits no trouble.
With their permission I've passed on this data to Lightning Fibre and am currently waiting for their next response.
Their initial response, while extremely quick, wasn't really inspiring. I was asked to use the speedtest.net app (Windows, Android or Mac) instead of the website as they said the website test can't generally keep up and showed me multi threaded test results comparing both the Windows app and the website on their laptop. I pointed out in my initial ticket message that it's single thread speeds as well as the result of that test. I also gave them the results I had on day one which were far better.
Unfortunately the speedtest.net Windows app doesn't appear to have an option for doing a single threaded test. Android however does so I plugged in their equipment temporarily and stood within a meter using my mobile to do a test, albeit wireless, and sent them the result of that test. Neither my system (an overclocked Threadripper 3990X 64-Core CPU) or the other Lightning Fibre's customer's system (an i9-12900K) are slow systems
.
I also sent Lightning Fibre some additional iperf3 test results using two dedicated servers both on different networks and both with 10 gigabit ports. The tests were performed between my Lightning Fibre connection and the dedicated servers. Unfortunately they also showed similar slow single thread speeds. I've checked if this is depends on time of day and it's consistently on the slow side no matter if it's early morning, midday, early evening or late evening.
So, from this I at least know I'm not the only customer who could be experiencing a slower than imagined single thread download speed. I know it's not my equipment and I also know their network locally is performing fine so it's likely a problem somewhere between their network and the internet (issues or congestion with their transit(s), an underlying fault somewhere or possibly a misconfiguration). I'm hoping my findings will eventually be passed on to someone who has much greater access to their network infrastructure, e.g. someone in their NOC, to look into this and hopefully resolve it.
It's not a deal breaker but it's not what I was expecting. Perhaps at peak times I would've expected slower single thread download speeds but not all day long after day one.
This is likely a good test run of how their customer service is. Hopefully a positive outcome will come from this!