Earlier quoted context omitted.
It's essentially imperceptible. We measured it, and our client-side script introduces a 0.1% CPU overhead.
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?
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.