try removing the battery saver on the app. by default, I think all apps will be placed under battery saver mode.
It is indeed under battery saver mode. I just changed its restrictions. Will see if it makes a difference. I dont rmb my note 4x lagging in battery mode tho. If battery saver mode is indeed the cause, then the note 9 is actually quite unusable by default since apps like IG will lag.
Edit: I searched online for users facing similar issues. It is indeed a bug but theres a stop gap solution.
Go to "Settings" Then "Display" Then "Notch & Status bar" Then "Hide Notch" Then select the third option "Hide & move status bar"
I tried it on the game Hill Climb Racing without changing its battery saving settings. It used to lag like mad but now its smooth. The only downside to this solution is that it makes your screen black at the top where the camera is. I guess have to live with it till they roll out the next update.
Edit 2 : IG story still lagging after a few swipes even without battery restrictions or doing the above method.