How does one climb up/down ladders the quickest?

The most efficient way to climb up/down a ladder is to face the ladder on an angle (so you are perpendicular to the ladder) and look straight up to the sky then use a combination of the up key with a left/right key to move diagonally on the ladder.

To climb up

Left arm closest to ladder - Strafe left and go forward (default keys: A+W)

Right arm closest to ladder - Strafe right and go forward (default keys: D+W)

To climb down

Left arm closest to ladder - Strafe left and go backward (default keys: D+S)

Right arm closest to ladder - Strafe right and go backward (default keys: A+S)