Job rate dropped dramatically on same backup job config but on virtual machine.

To all,

I have BE 2010 R2. On a phyiscal old file server, the backup job rate is 1804MB/min for 660GB in backup size. The job takes 7 to 8 hours to complete to tapes. It's acceptable to us. I then created a new virtual machine on VMware as our new file server and installed the backup agent in OS. I copied the files over to the new server, less than 400GB, however, same backup job to tape, it takes about 12 hours at a job rate of 560 MB/min.

Looks like the backup job on VM runs slower than on physical server. Is it normal? Anything I need to tweak on the backup job.

Please advise

Thanks
Calvin