The speed difference is definitely noticeable, the animations are a lot smoother, things load quicker even from initial launch if the application was written for watchOS two or three. Everything is just a lot snappier and smoother like to watch should've been in the first place. This is comparing watchOS 3 on the S0 to watchOS 3 on the S2.
How fast is Hey SIRI compared to series 0 with OS 3?