A porter is not going to get you checked in quicker regardless of the tip. All they do is take the luggage and load it in bins for transfer to the ship. This is before you get to the check-in area.
Check-in speed is a function of the staff, the number of passengers in line, or if you have priority embarkation (suites or platinum, diamond, diamond plus C&A member).
Tipping two or three bucks per bag is fine.