To remove the delay, open Terminal, type or copy and paste the following line and hit return:
defaults write com.apple.Dock autohide-delay -float 0 && killall DockTo restore the default behavior, enter:
defaults delete com.apple.Dock autohide-delay && killall Dock[kirkmc adds: This is a nice hint. It has always annoyed me that, when moving the mouse to the side of my screen (I keep the Dock on the left side), there is that half-second delay before it displays. I've applied this setting and will use this permanently. It's not that a half-second is a long time, but my brain expects the Dock to appear more quickly, and it is always stymied by the wait.]