You must not be using his d4 correctly. It is indeed not his fastest poke, but it is claimed that he has the best d4 in the game (I don't know if this is true). If it does land, you can dash your way in and start pressuring away. The d4 is not meant to be used to win a poking war.
d3 is a very good poke for Sub-Zero, actually. It helps him when he's under pressure and can really throw the opponent off.
His d1...don't bother with it. beef_supreme already worded it best, so I won't bother explaining why.