When you're multiboxing, you generally want the High Performance power profile in Windows, and usually this power profile is set to 100% CPU power by default. The link in the warning message takes you to http://isboxer.com/index.php/guides/...cpu-throttling, which demonstrates both selecting the High Performance profile option and checking that setting.

But to answer the question, if you're not in high performance mode you can probably expect lesser performance out of your system. That's the main reason for this message -- people especially with laptops would come to me and ask why their performance was so bad. The selected power setting was it most of the time. Now these people get a handy warning so that if their performance is bad, they know something that they can try.

Another related issue that used to be common (could still be) is if the CPU power is constantly being adjusted while playing, which causes this really fun stop-start-stop-start sort of lag (e.g. FPS limiter kicks in, which causes the CPU power to drop, which causes the FPS limiter to back off and use more of the CPU, which causes the CPU power to go up, which causes the FPS limiter to kick in, and so on).