Where to look if the site slows down?

Question

Nginx

Colleagues, good afternoon!
Please tell the noob what to do in such situations:
Installed nginx as a proxy server. Using the ip_hash method, it forwards client requests to 2 backend servers on which IIS is raised and the site is spinning.
The site is old, there is a lot of things wound up. After the next deployment, he began to slow down.
I did this: I looked at the load of the server on which nginx is installed (percent, memory, disk), then I did the same algorithm with two ISS servers.
It turns out that everything worked as it did, nothing was touched on the servers. So it's in the code?
What algorithm for identifying problems should an administrator have, or maybe there are some useful utilities, or methods for checking performance, based on their indicators, to prove to developers that the problem is with their code. You understand, the phrase "no one did anything" does not always work. Thanks for all the replies and good luck!

Answer 1

https://developers.google.com/speed/pagespeed/insights/

Answer 2

Check if the cache is spinning. Maybe some module was killed during the update or the settings were knocked down. In general, you need to look at the speed of the server's response to requests and there will already be an understanding of what slows it down - either the server itself or rendering on the client side.