Rust itself or the way the Rust logic is implemented is not the bottleneck. Like most decent web applications the bottleneck is the database and how the decentralized protocols themselves are reconciled there.

Scaling massive amounts of records like Lemmy has been forced to is almost always IO bound at the database level even when a web service is centralized; this is much more difficult in federated architectures. This is why “NoSQL” databases have increased in popularity, but they are also not a magic bullet as there are major ACID trade offs one needs to consider.

This isn’t “one or the other” IMO. There’s room for niche instances hyper-focused on a generalized topic like “math,” “comp sci,” “sports,” etc.

But then there should also be a massive generalized instance (hopefully 2 at least so the competition keeps admins in check) that has a little bit of everything and acts as a Reddit replacement. We can have our cake and eat it too.