This is IMO mainly because they've stuck with the main implementation of the homeserver (Synapse) being written in Python. Had they've gone through with developing Dendrite as the main implementation, or perhaps even gone with one written in Rust, all of this would feel much snappier.
In a way, Dendrite is now being primed to be compiled to WASM and used in a p2p setup of Matrix, but hopefully we'll also see it running on servers instead of Synapse one day.
Although, to be fair, you can't really blame them too much though, they're a casualty to the Coronavirus effects like a lot of similar services are currently.