I agree with Andrew. The Wi-Fi network operating as 802.11g is consistent with ~ 20 Mbps actual data throughput.
However, that doesn't explain why the ethernet connected speed is reduced. If the simple reboot (reset; power cycle) of the router doesn't help make sure you have the latest firmware. Then reset the router to factory defaults. If the ethernet connected speed is still significantly reduced I think that the router is defective. But it would be good to double check with another computer and ethernet cable if possible.
However, that doesn't explain why the ethernet connected speed is reduced. If the simple reboot (reset; power cycle) of the router doesn't help make sure you have the latest firmware. Then reset the router to factory defaults. If the ethernet connected speed is still significantly reduced I think that the router is defective. But it would be good to double check with another computer and ethernet cable if possible.