Thats a good question, The onboard charger will need to be lighter, so it may have a lesser charging rate, since its heatsinks are likely to be smaller.
A fast charger is the stationary thing that you get installed in the motorway service station car park...it can be as big and heavy as you like, as you dont have to drive it around.
onboard chargers are typically the motor inverter run in reverse - uses the on board water cooling and of course saves weight - as only one set of power electronics.
The DC Fast Charge station uses high-current DC for the charge, which can provide up to 350kW.
AC chargers operate from the main's power so are limited by what the maximum power the mains can provide (6.6kW for a 220V, 30A outlet).