Siege [0], is my go-to. Easy to add to CI, because its on most platforms, and has decent output configs. You can vary headers and pages to hit, and how long connections get held open, or just reject them. And of course, it has great concurrency.

It can bring down most servers that aren't configured to prevent DOS attacks, like naive nginx or Apache configs. Usually somewhere between 5-10mins.

[0] https://www.joedog.org/siege-home/