There's a difference between resource efficiency wrt scaling up vs a constant (small) scale.
What the fediverse could use (as you highlighted) is a resource-efficient server for small-to-medium scale, think max a few hundred users. While the official Rails app has problem, what Red Planet Labs has built is a JVM server that runs Clojure. That by design comes with a large memory footprint, which is not resource efficient if you don't have enough use for it.