FPS drops occasionally while scrolling through app, Freezing my iPhone

I’m having an issues with the fps, when scrolling through X app or what’s app I’m noticing very clear fps drop some times it stops for half a second and continues the scroll and when using some pdf files it some how gets worse it’s very annoying, brought the phone to an authorized apple service in my region checked the device and said there was nothing wrong with it and did factory reset and the problem still there, any help would be appreciated.

[Re-Titled by Moderator]