I did end up on this link https://www.sonicwall.com/support/knowledge-base/set-mtu-in-vpn-environment-in-case-of-throughput-issues/170705131319789/ but the command which is suggested do not seem to be valid on a Devuan GNU Linux system.
ping -s 996 www.yahoo.com
Ping itself gives the total count +28 bytes of IP and ICMP headers.
Why all of this? well, for instance 996(1024) is the maximum MTU on a roaming simcard over the WINDTRE network in Italy… 1436(1464) over PosteMobile (non-roaming). Since I do have an OpenVPN on port 443/TCP which accepts only connections from known IPs… well, I had to go over my german symcard’s network to reach home.
Now fiddling with link-mtu and tun-mtu… stay tuned