A weird issue: speed dropped

Here is my code:

import express from "express";
import postgres from "postgres";
const sql = postgres('postgres://test:test@127.0.0.1:5432/test')
const port = 3000;
const app = express();

app.get("/", async (req, res) => {
    for (let i = 1; i <= 20; i++) {
        const xs = await sql`select * from posts where id=1`
    }
    res.send(`ok`);
});
app.listen(port, () => {
    console.log(`Listening on port ${port}...`);
});

I run the following ab test:
ab -n 2000 -c 5 http://localhost:3000 ( number:2000, concurrency: 5 requests at a time)
The result is stable at around 900/sec, whatever times repeat the command.

Then I run:
ab -n 2000 -c 10 http://localhost:3000 (change 5 to 10)
The result is still around 900/sec

Then issue came. When I run
ab -n 2000 -c 5 http://localhost:3000 (change 10 back to 5)
After repeat this command several times, the results significantly dropped to 150/sec, and even keep dropping.
If I restart the express server, and run the above command, it become stable at 900/sec again.
This issue is that, if I set -c 10 and then set -c 5, the reqeusts/sec will drop significantly.

I have tested both ts-node and bun, both express and bun native serve, same issue occured.

Start by reproducing the shown Express/Postgres.js example with ApacheBench, especially the sequence changing concurrency from 5 to 10 and back to 5. Compare the behavior across the tested runtimes and servers, then determine what causes throughput to keep falling after the concurrency change; done means the cause is established and the regression can be prevented or explained.