I have a web site, which was powered by Nginx 1.7. The SPDY 3.1 was enabled. Later last week, I upgraded it to Nginx 1.9.7 mainline version.
HTTP/2 already built with Nginx from version 1.9.5. Why not enable HTTP/2?
I just did a very rough test.
1. Pingdom testing tool
SPDY 3.1: Performance Grade 97/100, 18 request, load time 3.17s
HTTP/2: 97/100 18 request, 867ms
SPDY 3.1: Pagespeed score A 98%, YSlow score A 95%, Pageload 1.0s,
HTTP/2: Pagespeed score A 98%, YSlow score A 95%, Pageload time 0.6s
SPDY 3.1: Grade F A A n/a C Check
Ref: Nginx HTTP/2 module