Hours and days?! I've certainly never seen that, that's a bug. The most problematic rooms (like Matrix HQ) took no more than 20 minutes for me, and that's only for the first join -- subsequent users get to join "for free".

Performance is going to be a major focus in 2022, with [fast federation room joins](https://matrix.org/blog/2021/12/22/the-mega-matrix-holiday-s...) and a new version of [the sync part of protocol](https://matrix.org/blog/2021/12/22/the-mega-matrix-holiday-s...).