The Apple Watch has pretty severe resource constraints to fit into such a small package. At the risk of oversimplifying things, current third-party Apple Watch applications are essentially remote iPhone displays, so they aren't going to perform amazingly well.
As developers learn how to work with this new platform best, things will improve. Also, Apple have already said that they are going to open up the SDK further to allow for applications truly running on the watch itself, which will be a big improvement. My guess is they'll open that up in a couple of months at WWDC.