We need to be more open about what security measures we have built into the service, I think. I can tell you right now that the free tests most people are running have a built-in cutoff feature that aborts the test if it detects that the server on the other end is starting to respond significantly slower than it did at the start of the test.

Example: the test starts by simulating 10 clients. The average page load time measured is 500 ms. The test then moves on to test using 20 clients. If the average page load time goes above 1 second (twice what it was originally), the test will be aborted.

We have a general security philosophy that is based on different levels of trust. An anonymous user is trusted the least, a registered but non-paying user is trusted a little bit more, and a paying user is trusted even more. The level of trust determines how often you can run tests, how much data your tests are allowed to transfer, how often you can place load on individual destination servers (IPs), what settings you can make for your tests, etc.

Thanks for the reply, but I still think there's a huge disconnect.

First of all, you're admitting that it's okay for anonymous people to inflict a significant, measurable delay in page load time for my entire website. That is completely unacceptable.

Also, while I'm sure your system is well designed, it probably isn't perfect (because nothing is). If your monitoring service malfunctions then the tests might never cut off.

There should only be two levels of trust: people who own the server (and can test it), and people who don't own the server (and can do nothing to it).