Should I use std::vector::at() in my code

Question

I noticed today that std::vector::at() is significantly slower than accessing values with square brackets []. According to the doc .at() i

Answer 1

The difference between at() and the operator[] is that at() signals if the requested position is out of range by throwing an out_of_range exception.
So with the at() you can react to the error state. Using the operator[] to access the vector out of index will result in undefined behavior.