If you moved a Node service from HTTP/1.1 to HTTP/2, when would that actually buy you something, and where would it not?
A good answer keeps application-layer multiplexing separate from transport-level head-of-line blocking, and knows which one HTTP/2 fixes and which stays.
The main improvement in HTTP/2 is multiplexing. With HTTP/1.1 a connection carries one request and response at a time, which is why browsers open several connections in parallel, and a slow response can hold up whatever's waiting behind it on the same connection. HTTP/2 lets a bunch of requests and responses share one connection as independent streams that interleave, so one slow response doesn't block the rest anymore. It also compresses headers with HPACK, and there's server push. If you're serving lots of small resources that cuts latency and connection overhead. Node has a built-in http2 module for this. I'd say the move usually helps when you've got many concurrent requests going to the same origin, because you get the concurrency without paying for several TCP and TLS handshakes.