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By the way, have you tried just reinstalling YouTube? I have an iPad 4 still on iOS 6 and both YouTube and Netflix apps still work (albeit, the apps haven't been updated in forever). The Netflix is good to have since Netflix on iOS 10 doesn't allow screenshots anymore. :p

Netflix still working on 1st-gen iPad on iOS 5 even.

I bet your iPad 4 on iOS 6 is faster in many ways than an iPad Pro on iOS 10.
 
Netflix still working on 1st-gen iPad on iOS 5 even.

I bet your iPad 4 on iOS 6 is faster in many ways than an iPad Pro on iOS 10.
UI is very fluid.

However, we have the following in our household:
iPad Air
iPad Air 2
iPad Pro 9.7
iPad 9.7 (2017/5th gen)
iPad Pro 12.9 (2017)

While the GUI on the iPad 4/iOS 6 is very nice and responsive, Air 2 and newer on iOS 10 are just as snappy. The Air, yeah GUI is notably laggier but even Air/iOS 7 couldn't match iOS 6 smoothness anyway. It just didn't have the GPU performance needed to support the given resolution coupled with the heavy eye candy. I reckon thermals and power consumption wouldn't allow Apple to do A7X in the thinner form factor.

Mind, all the iPads I listed (including Air) are faster for web browsing. Besides, plenty of webpages render weird on iOS 6 Safari.
 
annnnnnnnnnnnnnnd iPad Air wont get iOS 12 for sure, considering how slow it is already. Besided there are no more A7 Deviced on apple store for almost 2 years as they stopped producing it.
 
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