How is the server performing?
Browsers do give us some information. In particular, we discussed TTFB or Time To First Byte earlier. TTFB is mostly a measure of server performance, and if it is more then about half a second, something should be done to speed up the server.
All the browser lets us see about the server's performance is what endpoints are slow. Knowing which API endpoints or pages are slow gives us a place to start looking for what is causing the slowdown.
Another place we should start looking for performance bottlenecks is by looking at general server statistics. CPU usage, memory usage, and bandwidth usage are all areas to track.
In particular when checking CPU usage and memory usage you should review individual applications, such as NodeJS or any other application that may be consuming a significant portion of your processing power or memory usage.
If the server uses a database or accesses another server, you will want to check how those are performing as well.