I have been having this same sort of issue using iCloud to sync across mobile and desktop. The iOS app would often hang or crash on initial launch, and then “catch-up” and work fine one or two launches later. I recently switched to syncing with git following these instructions:
This seems to have solved the issue and the first launch experience is very snappy now. So, for me at least, this was probably an issue to do with scanning and indexing the iCloud files on launch. Hopefully the added overhead of pulling and pushing using Working Copy isn’t too tiresome, we’ll see.