@leocg Thanks, I had checked that all the time - and yes: plenty processes from opera (like 30+), consuming the full RAM. When I had limited the RAM available for Opera in GX, the problem came up earlier, as soon as the allowed RAM was used... Also I had shut down all other apps.

Since last year I had more and more problems with Opera using endless RAM. While 100+ tabs were no problem earlier, this has not been possible recently.
In Jan 23 I switched my computer to a new one, i7, 16 MB RAM (as before), Windows 10 pro, today Opera 97.0.4719.63. Same problem: with 50+ tabs everything becomes slow... even with 20+ tabs open the cursor is sort of "jumping".
Tried Opera GX, but no difference - the only effect: while synchronizing I lost lots of bookmarks. This is really a poor experience, Opera! I would love to use it further, but with this poor performance it is impossible, unfortunately.