0x

In the case of NodeJS, which use the V8 Engine [0], you have access to the diagonistics API [1] that allows you profile your cpu or memory consumption. There are some tools that make that easier (see [2] or [3]) but you often left to interpret the result yourself [0]: https://v8.dev/ [1]: http://nodejs.org/dist/latest-v12.x/docs/api/inspector.html#... [2]: https://github.com/davidmarkclements/0x [3]: https://github.com/vmarchaud/openprofiling-node PS: Disclamer i wrote one of tool i mentionned above [3]

I have a similiar problem like you, except it's Java and more like 15 years old... What helped? Using a debugger and stepping through the code was useful, it's more a less a REST-API here (build ontop of the system, before it was SOAP, etc.pp) and I've just used some heavily used endpoints and stepped through all the way... Another huge boost in understanding was using flamegraphs (not sure what's hip for nodejs maybe this? https://github.com/davidmarkclements/0x) This was really an eye opener because that app also used an external huge Java ECM and there was lot's of AOP magic, reading the flamegraphs and looking at the source was a big boost in understanding. It's also a really useful tool to get visibility for performance problems that are not directly visible in the code. If there are tests, reading them might also be worthwile. And t