From my own experience I would say it's everywhere. Here is the list where I noticed it just in last two days:
Google chrome - news feed, facebook
Safari - Facebook (comments)
Facebook app - While scrolling and reading comments
Settings - got lag while loading screen of battery usage (in last 24 hours)
Settings - while changing wallpaper (I just swipped back and it got lag)
Photos - I was randomly switching between pictures
App store - Just when you open it, first swipe is nearly always ****ed
Home screen - while switching from right to left and left to right
iCloud storage, Google disk and even Mega storage and many more apps where you can scroll down. If you are patient once it will get lag. You never know when it happens but it happens. And once it happens then it's okay until you close it.
I really believe that would be much easier to make a list where everything works properly.