Since I use hetzner for a few things thats why I used them for some tests and when I was testing my aaisp line, I already knew how hetzner behaved normally from the work I do on my servers and data transfers.
I tested your vodafone link and on my line its actually slower than hetzner, about 400mbit. Also ramps up slower. But to linode its very fast almost 2gbit/sec. You win some and lose some and I consider that very good performance still.
To give you feedback I tested on a uk linode instance as well for both the hetzner test files and my server, the hetzner test files did the same as what you described a initial slower throughput but then suddenly jumps up after about 5 seconds. However when I tested a file on my server it ramped up as normal. The feedback you gave me on my server, I consider reasonable performance from both aaisp side and my side.
Been honest I prefer to test using my server's as I control the endpoint. I have always preferred using performance from endpoints I control over third party speedtesters, with the third party speedtester I respect the most probably been tbb due to its single threaded aspect.