I actually thought that too, but I guess that's not the case. I helped write some HTTP2 frame-parsers for Haskell using attoparsec, but apparently it wasn't fast enough as the lib author later rewrote all the attoparsec code to use pointers to the underlying byte buffers.

https://github.com/kazu-yamamoto/http2/commit/0a3b03a22df1ca...

The stream fusion stuff is sweet, but not exactly unique to Haskell since any language with good iterator/generator abstractions have similar constant-time memory characteristics.