There are a number of things that affect the performance of the backup. I/O bottlenecks are the most obvious of these. It is possible to bottleneck on both the read from disk and the write to tape. However, to achieve the sort of backup rates required, while maintaining all the information on which file is backed up to what tape, requires a lot of CPU: therefore, it is also possible for the CPU to become the bottleneck. Remember that the backup may not be isolated, and there may be other processes running, such as batch queries or the warehouse manager building aggregates.