You are aware that beta code may well include symbol information and additional debug code? There could be whole swathes of code that don't make it into the release image, those extra cycles and the extra memory footprint could really change battery life so don't start criticising IOS 9 or Watch 2 until the release code comes out and we know for sure.
These betas are there to give developers access to new features so that there are cutting edge apps available making full use of the new platform, also to get some stability feedback along the way for the new features. Things like performance and battery life can't be known until the release versions are available.