Another issue after installing the El Capitan beta yesterday: Emacs became slow and had font issues. Recording my fix here for future reference.
Upgraded to Xcode 7 beta and installed the latest command line tools
The beta application is named “Xcode beta” so I deleted the original and told the system to use the new one. Updated the app using homebrew and emacs was back in action.
sudo xcode-select --switch /Applications/Xcode-beta.app brew update brew upgrade emacs-mac brew linkapps emacs-mac