I've presented a few variations on this talk about how you can use standard Sitecore development tools to help find performance challenges in your code. I've got copies hosted here, in case you watched and want to go back over the material, or if you missed it and want to find out what I said...
The most recent version was at SUGCON 2019 in London. That's the version I'd recommend if you've missed out on seeing it live. But there's info on two other versions here too, in case it's of use: It was originally
a brief talk at a Manchester (UK) user group, and it was expanded to be
a session at Symposium in 2018.