If a query is performing poorly, and you can't understand why, then that query's execution plan will tell you not only what data set is coming back, but also what SQL Server did, and in what order, to get that data. It will reveal how the data was retrieved, and from which tables and indexes, what types of joins were used, at what point filtering, sorting and aggregation occurred, and a whole lot more. These details will often highlight the likely source of any problem.
I wrote this book with the singular goal of teaching you how to read SQL Server Execution plans
It will explain, among many other things, the following:
How to capture execution plans using manual and automatic methods
A documented method for reading and interpreting execution plans
How common SQL Server objects, such as indexes, views, stored procedures, and so on, appear in execution plans
How to control execution plans with hints and plan guides, and why this is a double-edged sword
How the Query Store works with, and collects data on, execution plans
With this knowledge, you'll have everything you need to read the execution plan, for any query of your own, regardless of complexity, and understand what it does and what is causing the bad performance. It is still your job to work out how best to fix it, but your new understanding of execution plans will give a much better chance of success!