What is pessimization?

Question

There is a comment on the question Can the use of C++11\'s auto improve performance? that scored many votes and suggests “makes it less likely to unintentionally pessimize” as a

Answer 1

You could have just looked it up in a dictionary, such as this page, which says:

pessimize (verb):

(transitive) To make (something) less efficient, such as a computer program.