Neither this comment nor the article clearly spell this out: when you use the production version of React, much of the developer-assisting error checking is removed (not just propTypes[1], but many other error checks[2]), so you don't need to worry about the end-user performance impact of propTypes.
As an example of how much this helps, one React-heavy page of mine dropped from 1500ms to 300ms by switching versions, according to my browser profiler.
OP is talking about compiling react in prod mode yourself, having pulled the source as an npm dependency, as opposed to pulling in the pre-built prod version with a script tag.
See the npm section right below the Individual Downloads section you've linked to in [2] - this is the recommended, and more common, way to use react.
As an example of how much this helps, one React-heavy page of mine dropped from 1500ms to 300ms by switching versions, according to my browser profiler.
[1] https://facebook.github.io/react/docs/reusable-components.ht...
[2] https://facebook.github.io/react/downloads.html#development-...