Thanks for the feedback – and the good questions. Answering those implicitly solved the issue…
Ad 1.) Yes, I updated OpenSuse 15.6 prior to updating NoMachine (zypper refresh, zypper update). The update was pretty substantial as I hadn’t done one in a few months.
Ad 2.) Yes, I meant the headless start in Virtualbox (“ohne GUI starten”).
Ad 3.) I tried the normal start, i.e. I stopped the “headless” Linux and restarted it normally. The normally started Linux did then not show the lagging when remotely connection to it via NoMachine.
I then stopped the normally started Linux and restarted the headless Linux. And, what can I say – issue solved, also the headless Linux is not lagging anymore.
Thanks again for great support!