Why is it very slow if -c is too large when using apache's ab pressure test?

Use the ab pressure test interface. If -c 100 is used, it will be very fast. At 300, 500, 800 or even 1000, it will start to be slow. 90% or 95% of the time will be faster, but there will always be a small number of requests that will take a higher time. , causing QPS to become very low. Why is this?

In addition, I tried it under linux

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Use the ab pressure test interface. If -c 100 is used, it will be very fast. At 300, 500, 800 or even 1000, it will start to be slow. 90% or 95% of the time will be faster, but there will always be a small number of requests that will take a higher time. , causing QPS to become very low. Why is this?

In addition, I tried it under linux