I recently took on a legacy application. The client was complaining of site speed issues. Specifically on Saturday and Sunday the site would just crawl. For a few hours the site would even go down. I installed NewRelic and started looking for the problem. Within a few minutes, I found the issue. One page of the site that displays data from SQL is where 80% of the time is spent on the site. There was a particular stored procedure that was running. Upon inspection on the query I found that the SQL was using some odd way of pulling data, instead of using JOINS. Once I changed the SQL in the stored procedure (and tested the results) we launched it to the clients live application. Instantly, the web application became more responsive and within a week we had several compliments sent through a feedback tool. We went from a 5-15 second load time, to less than 500 milliseconds!
It’s crazy to think, that in our profession, how much of an impact a few words in a single query can have.