The correct answer is yes and no.
Yes, having a second monitor will reduce your performance in games. With a second 30" monitor, I measure a reduction of about 3.5%. I'd expect it to be a smaller reduction with a smaller monitor.
No, running monitoring tools will not reduce your performance in games, at least as far as you'll be able to tell. There is a small reduction of far less than 1%, if you take the average of several test runs (which I've done), but it's smaller even than the standard deviation between test runs.