although I had already gone back to simply installing & activating Free 5.0 and was reasonably happy, after having received all your suggestions I felt I owe it to you to at least give AIS one more try, and report the outcome back to you all.
So, as it meanwhile has become routine, I de-installed Free 5.0, used aswclear5.exe in safe mode, re-booted and installed AIS 545, un-checked all functions except file system, web modules and german language.
Unlike before, this time NO LAGGING was experienced, don't ask me why.
I then added via control panel > add/delete the behaviour module. Reboot. Still, no lagging.
Added the script module. Reboot. No lagging.
Added the network module. Reboot. No lagging.
Added virtualization. Reboot. No lagging.
Added avast! firewall. Reboot. Bingo! Lagging half a minute.
Deleted avast! firewall. Reboot. STILL LAGGING. Strange.
Deleted virtualization. Reboot. Now, no lagging any more.
Conclusion: The combination firewall / virtualization causes the lagging.
Being behind a router, I feel sufficiently safe to live without avast's firewall function.
For me, the problem is solved.
Thank you all for your suggestions and help.
Kind regards: Walter