Oh my god...
I gave it a try and enabled IPv6 manually in the AppConfigLocal and a) it works like a charm (at least for now, needs further testing, I guess)...
...and...
b) all my lags from the last weeks seem to be gone !!

(of course this needs further testing, too, especially at prime times...)
"Short" explanation: I returned to the game in mid December after a good 4 months break and experienced severe lagging in menues, supercruise, etc. Only thing I changed in between was an upgrade from 50 to 100 MBit with the same ISP (1&1, Germany). So I took a search here in the forums and found different threads about port forwarding, VPN tunneling and IPv6. There were some irritating things though, e.g. my router (Fritz! 7590) has UPNP enabled, but ED says it's not and there was no way to get around. So I tried forwarding port 5100 manually, well it worked but wasn't any better. So I was thinking about giving VPN a try, but wasn't too sure about it as my ISP uses DS-Lite tunneling...
So I remembered this thread and gave it a try and for now it works and my problems are gone. It's really astonishing how fast the game connects to the ED servers on startup now...
So thanks to all in this thread here and I'll report back after a while.