I play on an iPhone 8 and an iPad (2018). Both work completely fine for me; no crashing, no lagging (okay, maybe a little lag at certain times but usually when I'm on cellular instead of Wifi). iPad Air 1 pretty much won't run the game anymore... mine wouldn't load the game after the latest update. Now, not to say that there isn't something in the update that is affecting certain devices, but I also tried a friend's park on my new iPad (his kept crashing) and it worked perfectly fine. I had the iPad Air 1 for a good 5 years before deciding to replace it (while it still works perfectly fine for most things)... mainly because I needed more memory (16GB vs 128GB), being able to play DV was part of it, but not the biggest part since I had recently upgraded to an iPhone 8 (from a 6s).
iPhone 8: I would check to see if it's connection issue or running too many apps in the background. Sometimes the phone needs a reboot (it is a mini computer after all), try turning it off and back on (I would recommend closing all apps in the background first and waiting a few seconds before turning it back on). Otherwise it should be working unless there is something in the update affecting your device (I think BFS has acknowledged that there might be some issues and they are working to resolve).