Zyxel NR7101 low performance

You can simply write an OEM image to the "Kernel" partition using mtd.

Or you can write it to the "Kernel2" partition and keep OpenWrt in "Kernel" if you like. Then you can switch to OEM from OpenWrt using

fw_setenv BootingFlag 1

and from OEM to OpenWrt using

nvram setro uboot BootingFlag 0
nvram commit