Modern and powerful hardware is needed to run EXR smoothly. In general, at least 3GB of free RAM is required to use the app. For iOS specifically, chipsets that have the same or even better specifications than the Apple A12 are recommended.
Examples of tablets that run EXR smoothly are the 10.2-inch iPad Air and the Samsung S7 tablet.
It is possible that you experience performance issues when running EXR on hardware with inferior specifications. Learn how to improve the performance on the troubleshooting page.