Upgrading Opera is out of the question since I have to have the 'open tab in background' feature.
If you mean middle-clicking / Ctrl+Shift+click, it's available in new Opera.
I'm on Windows 7. I have 8GB of RAM. When this happens memory usage is around 60%. There are no extensions installed for Opera.
Maybe it could be a plug-in. Take a look at opera:plugins and disable the ones you don't really need. Update the ones that you use.
And the 64-bit versions of Opera 12.1x have issues with plug-ins hanging. Try the 32-bit version if you aren't already using it.
The new Opera >15 is much better at this because it runs each tab and plug-in in its own process, so IF one of them hangs the rest of the page(s) and browser UI are unaffected by the hang. They'll be deprecating the old NPAPI architecture and that'll make the browser even more stable and secure.
The middle click doesn't work on the sites I use. Only the 'open pop-ups in background' does.