I had issues about uicollectionview scrolling.
What worked (almost) like a charm for me: I populated the cells with png thumbnails 90x90. I say almost because the first complete scroll is not sooooo smooth, but never crashed anymore...
In my case, the cell size is 90x90.
i had many original png sizes before, and it was very choppy when png original size was greater than ~1000x1000 (many crashes on first scroll).
So, I select 90x90 (or the like) on the uicollectionview and display the original png's (no matter the size). hope it may help others