SQL Server Backup Process Tuning
CHeck to see if you are having the backups stored on the same physical location as your data and log files. If you are trying to write all activity to one disk, that is going to be a likely bottleneck.

Also, if you are using a 3rd party tool (such as Litespeed) for backup compression, there is a chance you are consuming more CPU than you expected, and that could also result in less than optimal performance. 

HTH