I vividly remember a case where a really simple page in a system had become incredibly slow - on asking what had changed I was told &quot;all we did was add a count of the number of users&quot;.<p>Turns out the back end (they were micro-services - even though the term hadn&#x27;t been invented at that point) didn&#x27;t have a call for &quot;total number of users&quot; so the front end code (this was server side HTML generation - 10+ years ago) was getting a list of <i>every</i> user in the system and iterating over all of them in chunks and counting.