I had been using a iPhone 6 with the app. I have only actually been in the parks with it for 2 trips. But I often use it at home to check things and get an idea for planning passes. In the last 2 months when I would open the app it would sit there for a while and then just disappear. I would open it again and the second time it would sit there a little while and open correctly.
Now, my iPhone 6 battery has rapidly been losing its capacity over the last 2 weeks. I decided to get a new one since I've had this for several years. I got a XS. Now the app opens right up very rapidly and works smooth as silk. So, its very likely a processor performance problem with older devices. Those apps are performing billions of operations per second (well, on the newer processors). So if your performance is several years outdated it will be sluggish and sometimes just crash.
It is a reality that as devices, computers, etc have gotten more powerful, the programs have gotten more complex and need that extra speed to just perform as intended. That eventually leaves the older hardware behind and its time to upgrade. Its not a conspiracy to make you buy new devices, but it is a choice of programmers to create more powerful apps that delight the customers.