for now I guess you can disable opera://flags/#extended-lazy-session-loading to get the old behavior.
Thanks for the trick.
I guess that on both cases the pages will not start loading exactly when Opera is starting.
If it's same on both cases then why make a change that uses more resource?
This old behavior is much better than loading all tabs gradually.
The introduction to 'Delay loading background tabs' in Opera was really awesome. It made Opera quite distinguishable from other chromium browsers.
I seriously don't understand how loading all tabs gradually is going to help with the performance.