Here's how to set up Gnome Panel to auto hide instantly without animations or delays...

First open dconf-editor and navigate to /org/gnome/desktop/interface and uncheck the enable-animations key.

Then go to /org/gnome/gnome-panel/layout/top-levels/top-panel (or similar) and ensure that auto-hide is checked, hide-delay is set to 0 and unhide-delay to set to 0.