The difference is that in HTTP/1.1 browsers tended to use up to six TCP connections per origin. On a site that loaded many resources (a.jpg, b.png, c.js, an API call, &c.) from one origin, they were multiplexed across six TCP connections, and so if a packet on one connection was lost, the other five would be unaffected and could continue at full speed.

In HTTP/2, all those requests are multiplexed across only one TCP connection per origin (each request becomes an HTTP/2 stream); so now when a packet is lost, all requests in progress are affected, rather than as few as ⅙ of them.

If you only ever make one request at a time, this difference does not affect you, because only one connection would be used under HTTP/1.1 as well.

But this case can make HTTP/2 significantly worse than HTTP/1.1 on low-quality mobile networks.