It's just based on my observations; I've not tried said extension. Opera has always had a reputation for fast loading and rendering and it's something they try quite hard to maintain. I think their engine is faster at rendering than Gecko, although computers are so fast these days anyway that it doesn't make a lot of difference.
It's the UI differences that are attracting me at the moment - Opera has an extensive system of keyboard shortcuts which are very useful for laptops, and I only own laptops myself. It's always been little things though, like the Notes facility - highlight text, right-click and copy to note, it saves it in its own little clipboard thing and you can read or paste that note elsewhere with another right-click. I find this very handy. (On a Mac you can just highlight text and drag it to the desktop and it will save a clipping, so I mostly use this on PCs.) Or moving back and forward using Z and X, or just with the mouse buttons. Or any of a bunch of other little things, none of which are killer in themselves, but they add up to an overall better experience as far as I'm concerned.
That's why I've never seen much different in speed between FF and Opera (infact by using the Fasterfox instead of manually configing things FF is now alot faster than Opera for me)...
