But...if you have a speed constraint over a waypoint (let's say to cross a waypoint at a speed of 250kts)...i think it is better to drop below the profile rather than selecting V/S to shallow the descent...because you need some more distance for the speed reduction...as the FMGC was calculating a speed reduction from 280 to 250 not from 320 to 250
Yes you will need more distance to reduce. However whether to drop down or not immediately would depend on how high you are.
Remember that the managed descent profile at high levels has quite some residual thrust. Going just for OPEN DES instead of DES will make quite a difference already. As pointed out by
FD.