It may be with the fps you are recording in. I found that if I downloaded the movies to my iPad I could use the video editing setting in my iPhones and change to speed at which it was playing. It seemed to put all of my videos back to normal. I could see where the video started in regular time and then went into a slow motion mode.
Has Apple slowed down my iPad deliberately? Let's get this argument out the way first of all. The answer is no. Apple has admitted that it deliberately slowed down older iPhones in order to avoid What to Do If Your Kindle is Running Slowly or Freezing Jun 19, 2018 How to fix a slow or frozen iPhone or iPad | iMore
Your iPad shouldn't slow down in the same way an aging Windows PC slows down, but it might still suffer from sluggish behavior occasionally. There a handful of things you can test to get it Why is My iPad So Slow? How to Speed Up a Slow iPad
What to Do If Your Kindle is Running Slowly or Freezing
Is Apple Slowing Down Old iPads too? - iPhone Hacks Jan 19, 2018 iPad Running Slow | Why Is My iPad So Slow and How Can I Dec 27, 2019 Common iPad Pro Problems, and How to Fix Them | Digital Trends Mar 26, 2018 How to Fix iPad on iOS 12/13 Running Slow