I’m a fan of El Capitan’s auto hide menu bar setting and wanted to hide my dock as well. However, by default it takes too much time for the dock to come up.
Fortunately, there are a couple of Terminal commands to fix this. Open Terminal and paste the following lines one by one and press enter.
defaults write com.apple.dock autohide-delay -float 0
defaults write com.apple.dock autohide-time-modifier -float 0.5
You can play around with the values and see what you like best.
By the way: you can also speed up the Mission Control and Exposé animations by using this command
defaults write com.apple.dock expose-animation-duration -float 0.15