Correct, assuming you did mean voltage at the battery terminals.

I'm pretty sure the N900 does have internal switching step down regulator to charge the battery. By doing this it can operate over wider input voltage range and also minimise power loss (=heat generation) inside the device. Typically step down buck converters are 90-95% efficient.

As shadowjk points out, you really should look at the power source Wattage (U*I) to determine how fast the charger can charge N900 battery. This works up to a certain point where there is internal limits in N900 that will limit the charge current to prevent damage to the battery and battery charging circuitry.