Why is my iPhone 5s kicking me out of apps?

I've had my iPhone 5s ever since July 20, 2018. I loved it.

----------------- A YEAR LATER ----------------

My sister loved playing 'Animal Jam - Play Wild!' on HER iPhone 5s and so did I. But she complained about it 'lagging' out of her game amidst gameplay. It's started happening to me recently. My OTHER sister has an iPad (7th Generation) and it works with flawless speed and such. It's not just AJPW, it's starting to happen to other apps as well. For example, when I hit the city button in Photoshop Express, it kicks me out. Apple, fix this soon. I hate it. We ALL do.