Q: Iphone se

I've been having my iPhone SE almost for a month and now it is laggy and when I go back to main screen from multitasking with some opened apps, at the end of the animation a one pixel wide line appears at the left side of the screen for like a half second(the color of the line depends on the color of the opened apps). This line sometimes appear all around the screen after going back from multitasking. Does anyone have any idea how to fix this? I even restored my phone with itunes. Benchmarks (geekbench, antutu) don't give less points to my phone than to other SEs.