It would be wise to pick a somewhat lenient default max concurrent streams, so that anyone who never thinks about it is protected. People who care are encouraged to override the default.

Many servers default to 100, since the RFC suggests it shouldn't be less than that. Some implementations stick to that, or even lower without advertising it, since they need to dedicate a thread per request, which hyper does not need to do. Additionally, the HTTP Archive stats show that p90 is around 170 requests per page.

In order to not necessarily inconvenience most hyper servers, therefore, I'm thinking of putting the default at 200.