I also recommend an APM tool. An answer people will probably dislike and this may sound careless, but for the large majority of SaaS web applications out there

I also recommend an APM tool. An answer people will probably dislike and this may sound careless, but for the large majority of SaaS web applications out there scaling horizontally in the middle/web tier behind load balancers and having a beefy DB server with replication to a stand-by is going to be the answer like 99% of the time. From a business perspective. when you are operating in the cloud, the cost of having engineers spend a lot of time trying to tune something to run optimally can be more costly than just adding more cloud servers or increasing the size of existing or both. This is time lost that those engineers could be building new features of a product in order to bring in more revenue. Time-box that analysis to like 1 developer day (e.g. equivalent to 8 devs in a 1 hr meeting) and move on with business work. Or just increase resources til the problem goes away, assuming you've solved obvious things like N+1's, etc.