You'll still see performance benefits of HTTP2 while supporting HTTP/1.1 content.
HTTP2 requests have (sometimes significantly[1]) less overhead, and most sites following good HTTP/1.1 practices still make many requests.
Open the network panel in your browser and view a few large sites. Despite minimizing requests with sprites, concatenation etc, most still make dozens of requests, with some large sites pushing over a hundred.
(for example, I just loaded a page for a single tweet on Twitter and it involved twenty requests, with a size over 2MB.)
I think your list of options are incorrect:
- You can optimize for HTTP/1.1 and it will be fast over both protocols.
- Or don't optimize, and it will be fast over HTTP2 and slow over HTTP/1.1. The main benefit is saving development time & complexity. This will not be a worthy tradeoff until HTTP2 is more widespread among your users.
[1] The reduction in latency can be massive, for instance, as the HTTP2 server can push resources immediately without waiting for the browser to request them.
