Just out of curiosity, what is the CPU doing during these backups? Do you see one or more cores at full utilization? If so, it could be that this isn't a storage issue at all, but rather your server is maxed out in terms of processor usage. Also, how are your RAM levels?

If you don't already have a monitoring tool, try installing something like SQL Monitor - you can install a free trial, and it's a good thing to have regardless. That will allow you to track performance metrics over time, not just CPU and RAM but disk latency, read and write times, etc.