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If you are on an heavy app like a game or even watching YouTube it does lag or when the phone battery is 1%. It seemed smoother on iOS12 but not 60 FPS smooth.
 
Try do it on maps or while playing YouTube

Smooth as silk on my X, running the iOS 12 GM Seed. Haven't experienced any lag opening the Control Center on mine since buying it, even iOS 11, and I use the CC all the time.
 
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