Opening a secure web connection has traditionally required several back-and-forth exchanges before any real data flows, and a single lost packet could stall everything behind it. QUIC redesigns this layer to set up connections in fewer round trips and to keep unrelated streams from blocking each other.
It also lets a connection survive a change of network, so moving from one link to another need not drop a download or call. These gains are most noticeable on slow or unreliable links, where the old handshake overhead hurt the most.