When you wack too fast on the same target (many times a second), you get the message to slow down.  When this is triggered too often, you may be given a captcha to ensure you are human.

When you get the penalty message, it ignores some of your attempts, so if you get that *a lot*, you are actually sending less overall requests than if you didn't get the too fast message at all. As soon as you see it, back off a hair, the best speed is like, riiiiiiiiight up to the edge of that message, not over it.