You need to give more details..
The express you must understand is limited by 100mbit ethernet port..
From over 100mgb when plugged directly into my laptop
The express cannot get over 100mbit.. ever. The port is only capable of 100mbit.. which means at full speed you will get 90Mbit.
The wireless on the express is also going to be limited to some extent. Especially the older one.. gen1.
You need to tell us exactly what wireless link speed you are getting as well as the signal level.
How old is the computer you are using? Did you upgrade (so called) the OS to Yosemite.. if so slow wireless is often the result. Yosemite seems to work poorly with the wireless cards in some of the older laptops.
So please fill in what you are measuring on.. what do the apple diagnostics show about the wireless connection.. this has been built into the OS since mountain lion.
About Wireless Diagnostics - Apple Support
Does the same problem exist on iphone or other products or is it only one Mac?
Please tell us actual Mac and OS it is running.
Also how the express is setup in the network.. that is vital to give you why speed might be hindered.