How does 30W adapter charge faster than 12W?
It's not necessarily marketing hype.

Say you have a battery that can be charged at 30W/12V = 2.5A (and allows it).


If the charger can only provide 12W / 12V = 1A, then a 1A charger
will only provide....1A.  
Using the 2.5A charger will of course charge at 2.5A.
Using a 5A charger will not provide more than 2.5A,
because the battery circuit will not permit it.


The takeaway here is that more current will be provided by higher-wattage supplies, if and only if the battery circuitry allows it.