Can't speak for Robyn author, but in my opinion it's not a valid comparison. The only thing Granian and Robyn have in common is the usage of Rust and PyO3, but that would be like comparing two projects using Cython.
Granian is just a server, and as such it is comparable to projects like uvicorn, Daphne, hypercorn, gunicorn, uwsgi. It can be used to run any WSGI or ASGI application out there.
Robyn is a web framework, and thus it does a lot more than Granian, and as such it can be compared to other async first web frameworks, like starlette, litestar, sanic, quart, Emmett, blacksheep.
If we're talking about performance then.. well it's still Python running the app code. There's no way to escape that.
If we're talking about features, ergonomics, etc. then again, Granian is just a server, so there's not so much to talk about :)