Even in the first beta, the performance enhancements are real. Numerous Apple folks, including Craig Federighi, have said that with WatchOS1 and 2 they 'overshot' how conservative they needed to be with RAM and CPU (out of respect for battery life), and with WatchOS 3 they have rebalanced that.
Time will tell how much of a hit battery life will take from this, but for a beta things look good so far.