I was starting from a CRUD application with forms for inputting all the fields to store in a database, and then a PDF could be generated off of that. So, I already had code to calculate all of the dependent values from the inputs. Turning it into react took about one days work to get the poorly performing prototype, and fully switching to ImmutableJS took another few hours.

Also, I know approximately nothing about web browsers. It's possible that using something like bootstrap or angular would have given me a big boost here as well, but the article is comparing to vanilla JS, not to other frameworks.