Your API Can Be Slow Even When Your Code Is Fast

You optimized your function.
It runs in 5ms.
But your API still takes 2 seconds.
Why?
Because your function isn't the only thing your API is doing.
A request might look like this:
Client
↓
API
↓
Database
↓
External API
↓
Response

Your code could be extremely fast while the database or external service is slow.
For example:
const user = await getUserFromDatabase();
const weather = await getWeatherFromAPI();

If the database takes 500ms and the weather API takes 800ms, your API is already waiting more than a second.
And that's before adding other work.
One simple improvement is doing independent operations in parallel:
const [user, weather] = await Promise.all([
getUserFromDatabase(),
getWeatherFromAPI()
]);

Instead of:
Database → 500ms
↓
Weather → 800ms

Total ≈ 1300ms

You can potentially get closer to:
Database ─── 500ms ──┐
├──→ Response
Weather ───── 800ms ─┘

Total ≈ 800ms

The exact result depends on the services and other work involved.
The important lesson:
Don't optimize only your code.
When an API is slow, ask:
"Where is the time actually being spent?"
Sometimes the slowest part of your API isn't your code at all.