There are a few reasons why large websites are slow:
The Internet Archive is slow.
We triple check every broken link to make sure that it is indeed a broken link and not a broken archive.org server. This means that especially websites with many broken links tend to be slow.
The archive blocks our IP if we scrape to fast. We are in direct contact with the Internet archive and they requested us to use a custom User Agent so they can track our behavior. They have the legal right to shut us down, so we have to be nice to them.
If you want to help the Internet Archive, then support them with a financial service, so they can invest in a faster infrastructure. They are good guys, who rely on open source techniques, which is unfortunately one of the reasons their speed is limited. We give a free recovery if you send us a screenshot of a donation upwards of $25.