Highlights include Sliding Sync (instant login/launch/sync), Native OIDC (industry-standard authentication), Native Group VoIP (end-to-end encrypted large-scale voice & video conferencing) and Faster Joins (lazy-loading room state when your server joins a room).
Covering up a bloated protocol by a faster language has its limits though, and in the case of Matrix, well, a faster language only buys you little