I'm having the exact same thing. Black lines, crashing app, slo mo during plays. Has been that way since the app launched.
Does this happen after you've been playing for hours straight? It might be heating up the device if played for an extended period of time which can decrease performance.
If this is happening since launching the game, it sounds more related to the device's ability to run the game based off it's hardware specifications. I have a tablet and my phone. My phone has seen better days, so I have to turn the graphics all the way down to the 2nd lowest in order to avoid the game from running in slow motion.
Under the in-game settings>Preferences there is a graphical slider bar. If you do not see a graphical slider bar, that means your device likely barely meets the minimum system requirements or below them.
Thanks for the response. I've adjusted all the graphics sliders and run it down to bare minimum to not overtax the processor but still have the same issue. It's really a rather random thing. It happens when I've been playing a while but I've had it happen after just being on for a few minutes as well.
The lines were something I noticed at launch and tried adjusting graphic settings but it never helped. They actually disappeared about 2 weeks in and now are back all the time. The slo mo during plays is a constant thing. It happens no matter how long I've been playing. I actually have certain plays that I know it's going to happen and have learned to adjust to compensate for it. I thought that it had to do with my iPad being a little older but now I'm seeing others with brand new units saying the exact same thing.