  The third version of the Hypertext Transfer Protocol (HTTP) is currently in
its final standardization phase by the IETF. Besides better security and
increased flexibility, it promises benefits in terms of performance. HTTP/3
adopts a more efficient header compression schema and replaces TCP with QUIC, a
transport protocol carried over UDP, originally proposed by Google and
currently under standardization too. Although HTTP/3 early implementations
already exist and some websites announce its support, it has been subject to
few studies. In this work, we provide a first measurement study on HTTP/3. We
testify how, during 2020, it has been adopted by some of the leading Internet
companies such as Google, Facebook and Cloudflare. We run a large-scale
measurement campaign toward thousands of websites adopting HTTP/3, aiming at
understanding to what extent it achieves better performance than HTTP/2. We
find that adopting websites often host most web page objects on third-party
servers, which support only HTTP/2 or even HTTP/1.1. Our experiments show that
HTTP/3 provides sizable benefits only in scenarios with high latency or very
poor bandwidth. Despite the adoption of QUIC, we do not find benefits in case
of high packet loss, but we observe large diversity across website providers'
infrastructures.
