I asked about that earlier today, and this is the explanation
It’s in Java, so there’s that overhead. But mostly, it’s less about “efficiency at all costs” and more about maintainability, being easier to contribute to / review, and having a less toxic development community. It’s got more developers working on it than Lemmy, and it’s in a language more people are familiar with (Java). It’s roadmap is also not constrained by the viewpoints of a small group of fairly, uh, controversial figures.
After the 1:1 compatibility phase is over, they’re both free to and planning to implement more features that the Lemmy devs either won’t or can’t be arsed to do.
The comment: https://lemmy.ca/comment/7677015
Yep, only the necessary data is federated. The other relevant data that's logged (which is much less than what other social media platforms collect to be fair) could potentially be abused