Open Terminal.app from Applications > Utilities Copy-paste one of the following commands & confirm it
defaults write com.apple.dock autohide-delay -float 0 && killall Dock
defaults write com.apple.dock autohide-delay -float 0 && defaults write com.apple.dock autohide-time-modifier -float 0.4 && killall Dock
Want to revert the behaviour and go back to adding a delay to show/hide the Dock again?
Apply the following command using the Terminal.app
defaults delete com.apple.dock autohide-delay && killall Dock
defaults delete com.apple.dock autohide-delay && defaults delete com.apple.dock autohide-time-modifier && killall Dock