Note that this add-on is unmaintained, and that its homepage (https://github.com/idlewan/link_cleaner) points to Link Cleaner+ as an active fork.

Does the add-on intercept and clean the URL before it is sent to the website? If so, does this affect performance?

A nice White Hat piece of script.

Thanks.
Does it raplaaces https://addons.mozilla.org/uk/firefox/addon/google-search-link-fix/ completle?

Makes links load faster due to less redirects. Highly recommended.