- Use the app, see what is slow from a subjective UX perspective
- Profile your code, find hotspots
- Use system monitoring to determine if there are any hardware/network bottlenecks
One thing I would add on these, perhaps a bit more specific: You mention you are using NodeJS, and it is a CRUD app. What is your database? Are you using an ORM? Sometimes the choice of especially an ORM can cause some performance issues behind the nice opaque interface. I have no specific reason to believe this is causing the performance issues for you, but I've had issues myself in the past so thought it might be a helpful suggestion to look into (for example, N+1 query issues).
- Use the app, see what is slow from a subjective UX perspective
- Profile your code, find hotspots
- Use system monitoring to determine if there are any hardware/network bottlenecks
One thing I would add on these, perhaps a bit more specific: You mention you are using NodeJS, and it is a CRUD app. What is your database? Are you using an ORM? Sometimes the choice of especially an ORM can cause some performance issues behind the nice opaque interface. I have no specific reason to believe this is causing the performance issues for you, but I've had issues myself in the past so thought it might be a helpful suggestion to look into (for example, N+1 query issues).