With latest update 2018.12, we've released a brand new iCloud sync. It's up to 10x faster than the previous one. Please make sure, that you update all your devices to the latest version. Due to technical reasons, we can’t bring these improvements to the Dropbox sync, so we’re deprecating it. If you’re using it, you can still sync to Dropbox, but we encourage all users to move to the iCloud sync.