Can you upgrade wifi card in airport extreme 2gen?

"Your AirPort is currently an 802.11n compliant unit.

Have you checked and if needed updated the firmware? Apple did make some speed improvements within the Extreme's firmware.

Otherwise you will need to review your network's design to see if one of the interconnections are running slow. Where are you encountering slowness, Local or the Internet via your ISP?"

I think by faster he means something 802.11ac or similar. I came here with the same question.