Especially well-optimized single-core smartphones handle everything that people typically use smartphones for with ease. Just go look at the various Windows Phone handsets.

One problem is most are going smaller, thinner, and sleeker. Thus the need for smaller batteries that won't hold as much even with improvements in battery tech itself.

For my trips I actually carry two detachable battery packs for my iphone since I only lasts a few hours under full use and the iphone generally has a good battery compared to other smartphones.