I don't think anyone is concerned about the CPU overhead. (Also, let's not talk about measuring things without defining exactly what was measured. On a blank page I'm sure your JS runs great.)
How does it affect network performance? If I click a link and it takes me to another page, how do you track that?
Good point. In terms of network performance, we try to batch events and minimize server requests.
For link tracking, our method is similar to how other analytics products track link clicks - we delay the page navigation the min of: 1) 250ms, 2) the time it takes for our server to respond to a click event.
How does it affect network performance? If I click a link and it takes me to another page, how do you track that?