First of all, the extension only runs when you are on Reddit or one of the shopping agent pages. If you are on another site, it's not slowing you down.
Either way, it is lightweight and quick, only 40kb in size. The link conversion happens offline and is almost instantly. The online information from RepArchive might need a few milliseconds to load, but while it's doing so you can use the rest of the site, including the converted links.