You forgot one important step I beleive.
Before deploying the app go to:
Heroku dashboard > settings > buildpacks > add buildpacks and then add github.com/mars/create-react-app-b...
Or, in command line you can do
heroku buildpacks:set github.com/mars/create-react-app-b...
If you don't do this step, heroku will deploy the development build (which is slow) of your react app instead of the optimized production build.
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