Restoring iPhone backup from iTunes taking too long
First of all, you need to make sure that you have the latest version of iTunes (Currently v12.4.2) installed on your mac so that it supports the latest version of iOS.
Secondly, you can't restore an iPhone with a later version of iOS (iOS 9.3.2) to an iPhone with an older version (for instance, iOS 8.x). So make sure you've updated your new iPhone 6 to the latest version of iOS which is currently 9.3.3, then try it again.

I recommend you read this link and also this one so you'll get more information.

I hope I could help you. Good Luck.