Q: Why direct ethernet connection gives 10 times faster internet connection than wireless via airport extreme

I have a cable modem that delivers 100 mbs download speeds when connected directly to my Mac Mini.  If I connect it to an airport extreme I get 10 mbs.  If I test the speed with an iPad about 10 feet from the airport extreme I get 30 mbs.  I get a transmit rate of between 78 and 100 on my Mac and an RSSI of about 73.  I have an airport express installed about halfway between the airport extreme and the mac. The diagnostics say the wifi is functioning as expected.  I am running OS-X 10.9.4.

What could be causing this loss in speed and is there any cure for it?

Thanks.