Besides what is already mentioned in sibling comments: QUIC requires one less RTT for connection establishment than TCP+TLS, which reduces the time to first by

Besides what is already mentioned in sibling comments: QUIC requires one less RTT for connection establishment than TCP+TLS, which reduces the time to first byte on new connections. Besides this QUIC allows the client to migrate between different IP addresses and networks. This means that e.g. a download on a phone doesn't get interrupted if the user moves from cellular to WiFi.