I have a 1st generation iPad that's been unused for around 4 years. I thought it'd be fun to downgrade it to iOS 4, and install the 1.0 versions of a bunch of launch iPad apps. I've got Pages v1.0, Keynote v1.0, Numbers v1.0, iBooks v1.1, and GarageBand v1.0 running on the iPad. And surprisingly, it runs pretty fast! Almost as fast as my iPad Pro on iOS 9, when running these old apps.