Send application by mail or made in person

Could someone explain me what the difference between send application by mail or made in person at Public Enquiry Offices (PEO), except of costs?
I heard that it affect on application processing time. Is it?

when you make it in person, it takes 1-2 days but in order to have an in person application, you have to have an appointment. generally appointment dates are very full, you need to book in at least a month earlier.

by post, it takes 6 weeks to 3 months sometimes.

if you have enough time until your visa expires (check the website to book an appointment) and money, apply in person.

You should only apply in person if your application is straightforward. If it isn't then they will not be able to make a decision on the day and your application will take just as long as if you had applied by post.