Very long backup runnin' time

Hello,

 We have 5 Windows server 2012 under Vmware.  Every day make a standard windows backup to an other - phisycal - windows server 2012 in two phase.

 Phase1:

 Server1, Server2, Server3 -  Windows Backup to \\backupserver\d

         - started every day at 6PM and finished before 8PM
         - on windows the backup server runnin' Backup Excec 2012 and starts at 8PM (LTO Tape)
         - backup duration approximately is: 1,5 hours
         - byte count is 296GB
         - post process clear the share...

 Phase2:

 Server4,Server5 - Windows Backup to \\backupserver\d

       -started every day at 23PM and finished before 1:30AM
      - on windows the backup server runnin' Backup Excec 2012 and starts at 2:30 AM (LTO Tape)
      - backup duration approximately is: 4,5 hours
      - byte count is 305GB
 

 I need help how to track down why the difference is so big between the two backup.

 Any help apperitiated.

 tnx:

Holian