QUIC is quite interesting https://en.wikipedia.org/wiki/QUIC

So.. Dutch ISPs - or someone else along the route - interfering with UDP traffic?

I suspect they are throttling UDP traffic on "non-web" ports, traffic might look like torrent encrypted stream :). Anyway you can test whats going on with https://github.com/lucas-clemente/quic-go

This is probably not the case, as traffic shaping is illegal in The Netherlands.

In a world where every router has the capability to do things which are illegal in various countries, do you think that Netherlands ISPs always obey the law? In the US, convention center operators keep on getting busted for sending deauth packets to "rogue" WiFi access points, i.e. those brought in by conference attendees and exhibitors.

The Dutch internet market is not really comparable to the US market. We do not have data limits for cable/dsl/fiber and I've never had issues downloading torrents at full speed, even before the net neutrality law.

I had issues with VPN and some dutch ISPs, so Im not sure about that.

They wouldn't be the first ISP to break the law, and they most certainly wouldn't be the last.