Reimaging ghost 11.x - unicast

I have a large scale network where we image lab PC's (win7) over the network.  If I reimage 3 PC's using unicast average time is 30 to 45min to reimage.  If I bump that up to 5 or more PC's to reimage the reimage process jumps to 4 to 5 hours.  I'm using a WinPE boot disk where it launches Ghost with no switches.  Is there a limitation on how many devices I can reimage at one time?  The images are on average 20gb in size.  Network is a gig network.  Images are stored on a network VM server. 

thanks

Winston