This is starting to look like a memory leaks issue. It’s when a code within the app (or a library it uses) forgets to release a portion of the RAM it has previously used but is no longer needed. The telltale indicator is that the bigger your memory, the longer it takes before it causes problems.
The more expensive phones have a lot of RAM. I was surprised to find that my Samsung has 12gigs of RAM. Even my old windows boxes had less RAM. And it takes more repetitions before the problem appears.
Edit - the workaround is to force close the app. That releases the unused memory.