Node.js is single threaded by default. A side effect of this is that your code could potentially be blocking the main execution thread. On a server that is shared by all your clients, this is really bad.
Your API could fail to take incoming request because it's blocked by CPU intensive work on the main thread. To avoid this, make sure any intense workload is asynchronous.
Blocking code
Here's an example of some code that would prevent your API from receiving more requests until its done.
import fs from "fs";
const result = fs.readFileSync("some/path/to/file.txt");
Reading a file with the sync version of the method is blocking. If that file was huge and running on a popular route where many requests triggered its execution, your API will eventually slow down. To avoid this, you'd want to make sure this code was async.
// promise version
import fs from "fs/promises";
const result = await fs.readFile("some/path/to/file.txt");
Now, this code will no longer tie up the main thread, allowing more requests to come through. If you ABSOLUTELY could not convert some sync code to async code, then you should use a child process to run the code on a different thread.