Unfortunately, this is unavoidable. In order to allow multi-photo selection (a very commonly requested feature) for iOS versions before 14, we have to use a third party library photo picker. It does seem slow with larger photo libraries, but I’ve looked and I cannot easily change the performance of this third party photo picker. The problems are architectural and fixing them would be a serious undertaking.
As you know, iOS 14 has an excellent native photo picker that is very fast and supports selecting multiple photos. If this performance is important to you, I encourage you to update to iOS 14 - it also brings many benefits like improved privacy controls over your photos.
I will look into the issue of changing photos in an observation, I’ve heard this from other folks as well.
Thanks,
alex