Thank you very much, Grandpa, for trying to help.
I carried out all your instructions (successfully) and tried to dowload another file. At first, I was really excited because it went at a cracking pace - over 70 KB/s - but very soon it slipped back down again and is now varying between 3 and 7....... And once again, all other applications have slowed down to a snail's pace.

Can you tell me, please, based on the information I gave about the speedtest and other stuff: should I be expecting anything else? Or is this as fast as I can expect?

Also, regarding the slowing down of other applications, is this connected in any way to the options for setting Download Bandwith? It is set at the default of 100% - is that why everything else is so slow? But if I reduce it, will that not just decrease the speed of downloads even more?

Thanks again for your advice.