Why do I have such a bad download speed on xbox?

I currently have a 70MB / s line but only 5MB / s, sometimes kb. I just can't use the console when it's snowing.

Have a LAN cable

Then it is probably because you live with several people. Or you live on a border, e.g. The Netherlands, because the Wi-Fi always switches between the Dutch and the German. If both things are not the case, it can also be due to the console itself.