You probably know that you can configure the Dock to hide itself when you aren’t using it. (If not: Go to System Preferences -> Dock, and select Automatically Hide and Show the Dock, or Control-click on the separator in the Dock itself and select Turn Hiding On.) You may also have noticed that, if the Dock is hidden and you then move your cursor to the edge of the screen to make it reveal itself, there’s a slight delay before the Dock slides back into view. The delay may not be that long—maybe half a second—but it does irritate some users.
If you’re one of them, OS X Hints reader
bahoom found that a simple Terminal command can make the Dock appear instantly. In Terminal, run the following command: defaults write com.apple.Dock autohide-delay -float 0 && killall Dock. That will change the setting and relaunch the Dock. When you move your cursor to the edge of your screen now, the Dock should reappear immediately.
If you ever want to restore the default delay, go back to Terminal and enter the command: defaults delete com.apple.Dock autohide-delay && killall Dock.