Rust vs. python might be part of it since python is basically designed to be quick to write. However, the lemmy devs, to their credit, are extremely thorough. They make sure that any feature they implement is fully available in the UI, the API, and has tests written for it.

For piefed, we test stuff as we work on it...but there isn't a formal test suite or anything like that. And, as evidenced by this post, many new features only show up in the web ui and may get added to the api later, time/effort depending. This is something we have been trying to get better at, adding features to both the web ui and the api at the same time, but it is more work so it goes slower.