Developing and maintaining any system requires many decisions about what to do, where to invest time and energy to get the maximum benefits fast. To make correct decisions accurate information is required. It is not a valid argument (although might be common) to say "I feel our DB interaction is slow, lets optimize our data access layer", or "To make our website super fast, I think we should start using CDN services". These decisions could improve performance for sure, but they might not be the best places to start. There is a difference between "feeling" and "actually knowing" something.