Short answer: No, not at all.

One of the most important priorities to us while designing Inspectlet is to ensure that Inspectlet is as unobtrusive as possible. Inspectlet's Javascript code is asynchronous, which means it does not block your site while it does its own work. Inspectlet isn't even loaded until the rest of your site has finished loading and is ready. Even after it loads, it stays out of the way and momentarily sends bytes of data to our servers. Try it on your site or check out the demo.