What's the model number? Check the link speed between the powerline extender. since it's AV500, it should be about 500Mbps. And I guess you have WPA4220 which comes with 100Mbps Ethernet port, so the link speed between the powerline should be at least 200Mbps. If it's lower than that, http://www.tp-link.com/faq-409.html

I have the WPA4220Kit and it's hard to believe that it would go that high. When I was setting it up in my kitchen with adjoining outlets it never went that high.

However, the thing that is really puzzling me is your firmware upgrade procedure. I used the tpPLC app to upload the firmware update into the wifi extender and it said it was successful. However when I click on the website icon, the webapp is still showing the old firmware version, 1.2.9 Build 151026 Rel.75883n. I then went to the powerline adapter and tried the upgrade there but it said it failed. Finally I went to the webapp for the devices and tried to upload the upgrade in the following order: the downloaded zip file, the extracted nvm and pib files. In each case the webapp popped up a dialog that said "incorrect format".

So tell me how to specifically upgrade the firmware to TL-WPA4220_V1_160421 so that I can improve my speed.

Thank you.

upgrade via tpPLC Utility, refresh it(it may have some delay in the interface).
For use the device, does the kitchen have a microwave oven? it may influence the wireless signal. Also, avoid the interference in circuit.

Days later the utility is still showing the old software as being installed.
Obviously refreshing doesn't work and you still didn't respond to my concern with the problem upgrading through the web interface.
I need a better answer than the one you gave.

contact with the support, i think. if the version haven't changed, maybe you haven't upgraded successfully.