How does a person get a dog to walk slowly?   All the tips on this site are intended to achieve results regardless of whether the dog is on or off the lead.   You should have a verbal relationship as well as a visual one with your dog.

What is the difference between the 'wait' command and 'slowly' command?

You must make your commands fit with what you would say to a 2-3 year old child if you walk in the town without a rein.   You want the child to learn the difference between your instructions.   It is no different from training your dog.

Most commands are negative but 'slowly' is positive as you need the dog to continue.   So when you give the command it must be a positive inflection in your face and voice.   A higher pitch than 'wait.'

It comes gradually and you must be consistent.

In town when Bonnie comes to a corner between two streets she knows that 'slowly' means meander to the corner and stop.   Any other instruction such as 'stop' or 'wait' must be avoided in the process of training.

When you have mastered the command you can use it when you are doing doggy activities.   We have a park about 12 miles away which has a dog adventure, so the 'slowly' command when learnt can be used freely in this situation.