I want to disable some of the Jetpack modules such as Stats in order to check if they make the web site slower or not. But I couldn’t see an option to disable modules one by one.
Of course, one can disable the Jetpack plugin as a whole. Is there a way to disable Stats module only?

If you press the "Learn More"
button for each Jetpack Module, you will get a "Deactivate"
button showing up to the right.

There is also a fork call Jetpack Lite
http://wordpress.org/extend/plugins/jetpack-lite/
A NOT official “lite” fork of Jetpack plugin containing only Stats and
WP.me Shortlinks modules. All other modules removed (files and code).