We had a large size environment with around 25000 nodes at 27 sites spread across the world, and we were deploying Windows 7 using WDS + MDT 2012. We found that deployment was taking around 4 to 5 hours, and also noticed that bulk deployments were resulting in outages and choking of the network bandwidth at some locations. Also, on injecting any driver or application at some locations, Deployment Share drivers and applications were not installing as a part of the OS image.
After a lot of troubleshooting, I found that I had mistakenly copied the boot.wim file from one deployment share to the problematic location, and forgot to change the deploy root entry in BOOTSTARP.ini file. Due to this reason, deployment was happening over the WAN link. The same reason was also responsible for delay in deploying image, outages and drivers and applications not getting installed on injection. I just changed the deploy root entry in bootstrap.ini and the problem got resolved.
OS Installation was taking a lot of time, outages, applications and drivers were not installing.