The iPhone 4 was optimized for Era because it was the newest phone when the game released. The game hasn't been optimized for new hardware/software since iOS 8, so of course the experience from anything from the 8+ to the X is going to seem horrible in comparison. Just because the hardware is newer (and just because you used to play on the 3G, a phone that was released in 2008) doesn't necessarily make the gameplay experience better for everyone
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