First sync is always a little slow, it's fine.

I wonder do you use this OneDrive for anything else rather than Joplin sync?

Does it have any sync client installed on any computer/phone?

Does OneDrive have enough free space to fit the photos? I remember it's only 5gb for free account.

If you got time you may try moving to another cloud (say Dropbox if your database is no more than 2gb) for now just to see if it makes any difference

Don't forget to keep a fresh backup before moving though