In reply to Emily.

Great question! We recently ran some tests to benchmark loading times in different scenarios. The tests showed that there is a slight increase in loading time when Jetpack is installed with no modules enabled. You can read more about the tests on this BruteProtect blog post. I imagine that in most cases the increase in loading time is negligible. And it could be offset by modules like Photon, which will increase performance, as well as modules that could replace additional third party plugins.