I just installed Opera 12.17 to verify your claims. I was very disappointed. This reminds me of people that say their old Nokia phones were better than the smartphones of today. Funny how time distorts things.
You probably just don't appreciate the extra features Opera gives. Which, of course, is fine, but for many of us they are a dealbreaker. I'm still looking for another browser giving me real MDI with "click tab to minimize" functionality... Otter Browser kind of does but falls short in other respects. Vivaldi doesn't even try (they haven't even made the menubar native...)
By the way, if you are referring to speed rather than features, you need to take into account that when you try Opera 12.17 right now, you are trying a browser that has been frozen in time for years while the JavaScript on websites evolved. For this reason, it's now very slow (if not incompatible) in JavaScript-heavy websites like Facebook, Gmail, etc... but I can attest as a long-time user that it was indeed the fastest browser in its time.
