Exchange 2007 backup is slow

Hi,

We are having one exchange 2007 server with multiple storage groups and database, a total of 1.8 TB data and I am using backup exec 2010 R3 SP2 with all patches. Initially the job rate was more than 3 Gbps and jobs sucessfully. Very recently the backup job rate has reduced to 1Gbps and the job is taking more 24 hrs to complete hence backup jobs is getting cancelled due to timeout.

Kindly provide some solution if any one have

Thanks and Regards

Moula