...it really depends on a lot of things:
* Speed of the NICs on the server & appliance set to the fastest speed they can manage (1GB FULL) along with the corresponding switch ports set to the same speed;
* Time of the backup when it runs - it shouldn't clash with any maintenance tasks such as AV scans or Exchange maintenance tasks for example;
* Type of data being backed up;
* How many jobs you're running at the same time to the appliance. More jobs would get better results as you're not backing up servers sequentially for example.
* Types of switches, server hardware etc.
Does this happen for all servers, or just some of them?