Exchange backups going very slowly

Our exchange backups are taking a very long time to complete, almost 48 hours at a throughput of 135 mb/min. we are backing up the information store. we have exchange 2010 running on a ibm server with 2008r2 OS. backup server is a ibm server running 2008r2 OS with backup exec 2012. i did a one time system state backup on the exchange server and it went fast(1400mb/min). Why would the system state backup go fast and the information store one go slow? The settings are the same, the only difference is whats being backed up.