Unfortunately it is becoming increasingly difficult to include new features and improve the performance of FulGaz while still supporting older devices and outdated software versions.
Many older Apple iPads and iPhones will struggle to run the latest version of FulGaz smoothly.
First of all, see if you can update your device's software.
The current version of iOS software is version 14 so if you are still on iOS 13 or below, you are several updates behind. Installing the very latest iOS version might help your device to run FulGaz with more stability, but many older devices will simply struggle with their lower processing power.