Hmm. Hard to say how it compares to Firefox. They both load in about the same time, but everything else (loading, browsing, interface) in Opera seems a bit more faster.
Memory usage is around the same, although Opera uses a lot less CPU time on my box than Firefox does - but that's just becuase my box is old.
As for features - Opera has a lot of them, Firefox not so much, but Firefox is every expandable. The current Opera email client also doesn't support HTML emails yet (due for version 10), which can be a disadvantage (I don't like HTML email, so its good).
Overall, I think its worth trying. I've used Opera since version 8 - which is was 2 or 3 years ago. Opera 9.5 is a vast improvement.
C. P. U. Its not a big processor... Its a series of pipes!