Not a performance problem, just one of convenience, you might find and one of many dynamic DNS providers on the net can resolve that for you.
Yes you would and… Depends. Personally I prefer the older image as it’s a full Ubuntu server system ported to ARM. The new image is snappy core and offers nothing for anyone who wants to tinker and tweak a system (and gaining SSH Access is a pain).
Snappy install mind, can’t really do anything with it.
I know your feeling. However consider what the pi is - a very small, underpowered compute card. Nextcloud by comparison can be a bit resource hungry especially with media files. How much data would you say you’ve copied over?
I’d say this will fix itself in due course given it’s just queueing io currently. How does it look now? Providing you don’t rinse it with data (which is always what we tend to do with a new box I know) it should chug away OK. Apps like preview generator will help but I don’t recall how the cron would be configured with the snap.