I have an Nginx proxy server. When an HTTP/2 request comes to the server and does not find anything in cache, the server makes an outbound request to the origin server using HTTP/1.1. Is there a performance degradation on the server when it converts from one version of the protocol to another? How does this compare to HTTP/1.1 to Nginx and HTTP/1.1 to the origin server? Is there a way to measure the overhead?
Strictly speaking there is performance degradation, since one protocol is binary, other one textural. So proxy must convert, that takes resources, time - you can expect degradation by default.
In general however that can be much more complicated. Say your proxy is used by slow mobile connection. Who cares about a bit of conversion if your app is gaining huge bust after that conversion? Or maybe your proxy had gzip conversion for http/1.1, and that speed gain is not that big, on the other hand maybe performance degradation is not that big?
Can you measure that? Perhaps. Question is what for? I would measure something as close to real case as possible. I would automate that measurement to see where real performance is.
My only warry in your case is CPU of the proxy - just measure it to see changes over time, and setup notifications - like "if cpu is over 80% for longer then 5 min".
Other than all of that. Http2 brings two ways communication, as well as push. My assumption is that it is not your case, since you are comparing 1.1 and 2. For me I would go with http2 everywhere, unfortunately nginx is not supporting http2 and backend side. Fingers crossed to see that soon!