Looking at your video I can see you're creating a document that is 2000x2000 points at 400 DPI. Did you want points or was you mistaking that to be pixels?
The document you are creating when printed is around 2.3ft x 2.3ft which is a huge amount of pixel data. Unfortunately the iPad 2018 only has 2GB of memory so it is struggling with such large amounts of pixel data when using pixel brushes as its really at the upper limits of what the device can handle.