Hey Stella, depends what kind of solution you are looking for!
You can try to limit the data synchronized to the server if you want to speed up the process, this can be done by only selecting the entities that need to be synchronized instead of the synchronize all option, or to limit the amount of data the user has on their device using Xpath under the Navigation > Configure synchronization.
If this isn't an option you can show the user a loading screen with a message that the device is synchronizing, when opening the app this can be also done in a splash screen.