It's just the way renpy games behave on older devices with insufficient processing power. Try manually shutting down anything working in the background. Although it's comparatively light and well optimized game. Maybe renpy vns are just not for your device?
I agree with above. I personally have a Samsung A11 phone for my everyday life stuff and this phone cannot keep a VN open for more than 10 mins. And not even the heavy ones too. It usually means the phone cannot handle it so it just shuts it down.
