Why do some VPN clients slow down the network connection even when they are not it use?
I can confirm by empirical testing that those VPN clients install a network driver which gets automatically enabled on each and every network interface.
Disabling this driver in the NIC properties (on NICs which are not related to that specific VPN) fixes the issue, and the VPN client still works.
I'm not going to reverse-engineer that, but at least this got rid of that awful speed cap without uninstalling the VPN software every time.