The time it takes to complete a sync depends on the amount of data that needs to be downloaded from Box/Google Drive/Sharepoint/OneDrive/Salesforce and the speed of the internet connection that your device is connected to.
It is usually the first sync that takes the longest as Crescendo downloads all your files. After which, all subsequent syncs will only download new or updated files.
We recommend that on your first sync, or when you expect a large download, you’re connected to a fast and reliable internet connection and that you allow Crescendo to download data uninterrupted.
Read this post to learn more about sync.