Sure, Element syncs much faster than other clients (FluffyChat is unusable for Instant Messaging) but leaving the browser tab open still thrashes one CPU core.
These shortcomings bug me like hell, too. I tried to overcome these by building https://fedi.buzz but the Fediverse community is quite split over its usefulness or it being a threat to their privacy.
FWIW, the contemporary alternative to 9p is virtiofs. It is implemented as a separate service that is running aside the hypervisor. Originally out of qemu, it was rewritten in Rust as a freestanding project that can also be used with cloud-hypervisor and crosvm.