Old Dell running faster than new MacBook

I believe it is to avoid drawing more power than the adapter can deliver 
(with the battery in, much like a Prius, the battery can help to cover 
spikes in load).

This sounded a bit strange to me since the adapter can power the laptop 
& charge the battery at the same time. Doing a bit more digging, it 
turns out to be correct.

http://support.apple.com/kb/HT2332 gives the official explanation.

Francois