Hi marten71
Welcome to the site.
This unfortunately is something we are seeing more and more of from all countries and its down to your ISP. The transfers start fine indicates the ports and firewalls are configured well. The speeds usually start fine, then your ISP shapes or throttles the transfers of Winmx and other p2p systems down till they choke and die off. Its very frustraiting for you and for us, as there is no good advice except to find a new ISP usually.
You can try port changes to see if this helps, most companies started off using software that shaped speeds or throttled by ports, but most have moved to more advanced software to do this that detect packets. For most port changes will not help but its worth a try.
https://www.winmxworld.com/tutorials/changing_port_numbers.htmlI would suggest numbers either very high or very low to test. Be sure to also make any changes needed in your Modem, router, or firewall if you set ports previously in those.
Useable ports are within this range 1 - 65535