Just so I understand the scenario, you have the same processes running between 2 2008R2 servers and 2 2012 servers, where the source and target are the same database versions. If that scenario is true, there isn't anything related to SQL 2012 that would make the backup / restore process take longer. The issue is more likely related to the hardware that the SQL 2012 servers are running on, more specifically the disks associated with those servers. If you're using DAS (Direct Attached Storage) on the SQL 2012 servers, ensure that the raid controller(s) are using a write-back cache setting and not a write-through. Older IBM servers used to automatically ship with the battery backup unit for the raid controller, the newer servers don't. It's an additional option. I'm not saying this is you're problem, but it's a good place to start.