Weirdly enough my dad and I own the exact same phone (iPhone 8S) while he is on the newer iOS I am still on the older (ios 12) for me the app loads within 5-10 seconds for him it’s upwards of 20-30 seconds as well!
It seems like a bug maybe in the app itself, or atleast hasn’t been optimized for the newer ios…
As I always like to say… if it ain’t broke… So I’ll be sticking around ios 12 for now