I ran a stressed app some years ago. We only had a wee little backend because our revenue was v.low, but we wanted to do stuff like sleep inside and eat, and so were motivated to cut costs to make profit.

What I did was make a table of all the queries that were being run on my backend, and I ordered them by the number of times that they were called and the cost of calling them (I honestly can't remember the measure I used for that but it was like cputime*memory or similar). I then did two things for the top queries.

1) Optimised them where I could.

2) Looked for where they were being used and tried to stop it.

(2) was very successful.