My longboard is slow, what should i do?

Just by reading what you said, I knew automatically what's wrong. It's your wheels. Poly-urethane is used for regular skateboards, and you don't go fast with it. You need downright urethane. Urethane wheels usually feel a bit squishy (not a lot though), and they go REALLY FAST. If you're wheels aren't squishy-like, and are rock hard, consider getting urethane wheels. if you DO have soft wheels (that's the same thing as "squishy") they might be really tight. Try loosening them up and spraying on some WD40. I hope this helped