Hi, Olivier!
A:M is much faster today, largely because we have faster CPUs and more available RAM and A:M has been optimized in recent versions to take advantage of the advances in these.
The benchmark scene that took 20 minutes to render on the computer I had in 2009 takes only 2 minutes on the computer I have now, which is still about four years old.
Also, NetRender is now included in A:M so you can take advantage of multiple cores on one computer or across your home network when you are rendering more than one frame.
A:M does not devote more than one core to a single frame nor does it use GPU-computation. Not yet anyway. A:M will be slower at rendering than some other programs. That is not a deal-breaker for me because A:M is better for me at modeling and animating.
Do i want to give up the better modeling and animating environment i get with A:M just to have faster rendering?
No! (en français!)
Rendering is something the computer can do while I'm doing something else. Modeling and animating is something I have to do at the computer no matter what program I use so it is better to use the program that is better at that.
That is why i use A:M.
If you have more questions, do ask!