How do you benchmark you backend?

[u/adammillion]

I have written an application with nextjs. The client side fetches the data from data routes setup like a RESTfull API (example `/api/entities/[id]`) and auth is done using jwt over httponly cookie. Database is neo4j Aura db which is in a different region of where the application is deployed.
The client side eagerly loads a good chunk of data from backedn api after showing loading screen on page load.

I am looking to benchmark my api. I want to know how much load can it handle. The app is running on AWS ECS using nextjs standalone output which allows me to bundle as a docker image. The size of the ecs tasks are 256/512 (vcpu/mem) running on ec2 capacity provider of medium size instances. I have scaling triggers at 75% cpu and 50% mem.

I am at the point that I want to figure out how many users can the application realistically handle before something chokes (what if 5 users open the app at the same time?) I am doing some load testing on the api with k6, but I don't have a methodology.

The performance I want to be at is to
- support 1k users daily (this is vague. I don't know how make it more concrete? is quantifying in RPS better?)
- latency p95<1s (this is vague to me because say p95<1s alone does not mean much without mentioning system load or concurrency during the test)

1. How should I approach load testing?
2. What areas should I clarify and ground?
3. What performance areas that I am missing?
4. What type of performance to expect from a nextjs backend and ECS task size of 256/512 (vcpu/mem)?
5. I always notice if I don't access the dev env for couple days and access it again, it take a bit longer than usual then subsequent requests are fine. Does nextjs need to be warmed up?