Slow Touchscreen Response on Ipad Air 4

I just bought my Ipad Air 4, and immediately applied a screen protector last week. It was smooth for a week, but it started delaying or didn't response when I swiped up to homescreen recently. I don't have or use many apps at the same time. I'm only using 22 gb. I took my Ipad to an Apple technician, but he said it was normal and couldn't fix it. Does anyone have the same problem with me? How can I fix it? Thank you in advance.