Make sure that you're using USB 3.0 (xHCI) ports for Windows Phone erasure. Erasure using USB 2.0 (EHCI) ports can be much slower.
How to check if the port is USB 3.0 or USB 2.0:
- When a device is connected, the USB ID is shown in top left corner of the icon. The first number of the ID identifies the USB bus that the device is connected to.
- USB bus information (xHCI or EHCI) can be seen in Settings > Host