tl;dr: HTTP/3 is UDP-based, rather than TCP-based, thus connections are established faster. It’s based on Google’s QUIC protocol. Use HTTP/3 if available.

Not all browsers support HTTP/3 yet, so be mindful of that. Check the latest status at Can I Use HTTP/3.

YouTube notably uses HTTP/3 on Chromium-based browsers (Chrome, Edge, etc.) to deliver a blazing fast UX.

Cloudflare ran some benchmarks showing that HTTP/3 is ~12% faster than HTTP/2.