Insert the usb drive on the working firmware, login ssh, enter the following commands:
cd /tmp/mnt/sda1 # Go to the root directory of your usb drive nvram show | sort > ./1000.txt
Then re-dump nvram on the buggy firmware
nvram show | sort > ./500.txt
Now compare the differences between the two files
diff 1000.txt 500.txt
Then you can try to find out which nvram variable is affecting the speed, maybe you can save a lot of AC68U users before the official solution is provided.