What is the fastest way to remove scale, hard water residues, rust, etc. from urinal and toilets?

Fastest way there is to go with strong acids, like ABC. If a customer had Super Ox on hand they could do that as well. If the residues are not excessive (like maybe in a shower) use of a lesser powdered acid solution (like Scale Away) would be in order. The downsides to use of strong acid solutions is that they do give off fumes (especially in humid conditions) which are very annoying and potentially dangerous. Obviously they are also dangerous to get on your skin and eyes. But they are the fastest way to go. NOTE: Always wear appropriate Personal Protective Equipment (PPE) when working with strong acids and read the label and SDS before use.