You can alleviate slow or reduced performance of backups involving network drives with a few simple steps.
ZCB allows you to create as many backup sets as you wish. Instead of one very large backup, consider splitting your data up into multiple, smaller backup sets.
There are many benefits to this approach:
If the source and/or destination drives are being accessed by other programs, it can cause performance issues during backups. The heavier the load, the slower the backup will be.
Whenever possible, schedule your backups to run at times when disk activity will be at a minimum.