The overhead for a simple textarea seems significant. On Firefox there are stalls (up to 500 ms) if I type "fast". I've tested the form on http://redux-form.com/5.2.5/#/examples/simple?_k=8ukhm8 (specifically the textarea).
Confirmed; just tested on my machine (Firefox, i7-3632QM @ 2.2GHz) by mashing on the keyboard. CPU spikes to 100% on one core and results in stalling.
In contrast the React+Redux app I'm working on uses the local Controller/Component state method and mashing on the keyboard there gave ~20% CPU.
I never would have imagined that Redux Form would lag on a desktop/laptop. I was more concerned that mobile performance would suffer. It's kind of impressive that a simple form can lag a desktop browser...
Here is the CPU time graph during my tests: http://imgur.com/we9uGQ7
EDIT: OK apparently it's not accurate, see comment below.