Sorry, can you clarify -- are you trying to use Smart Switch to transfer data from your S5 to a Windows 10 Mobile phone? Or are you using it to backup to a Windows 10 PC?
I have the same problem. I can back up to a work PC that runs W7 but at home on a W10 system I get to contacts and it just sits...forever. Unfortunately I can't offer any solutions. I have tried uninstalling and reinstalling the Smart Switch software.