Thanks, Akitori, for the hint.
I also had problems with the new release (17.4.2294).
First, after the first reboot after updating via GUI from the previous version (17.3.2291) the GUI was laggy and Windows (w7) a bit, too. So, I rebooted again and everything seemed fine. So I, unfortunately, deleted the offline installer of the previous version.
Second, one day later (May 4th) I noticed that the framerate was on average only 33% of normal values. Reading about others experiencing problems with games and such I uninstalled avast to check. Normal uninstall ("Programs and Features") didn't do the trick, I had to use avastclear (in windows safe mode) to get normal framerates back.
A re-install of v17.4 showed me that it in fact was a problem with the new version because the framerates "dropped" again by about 66%.
So, I used avastclear again and installed v12.3 (the latest offline installer I had apart from v17.3) and everything was fine.
Today, I read your hint, opted for the update nagging of the v12.3 GUI and it really seems that this problem got fixed.
So, thanks again for the hint!