Ok, a couple of things to consider:
- Based on the MCS Index value, the wireless connection between the Mac and the Express is using two spatial streams which gives is a maximum data link rate of 104 Mbps (2.4 GHz) or 240 Mbps (5 GHz).
- The WAN-to-LAN throughput perfomance of the Express can only support a maximum of 93 Mbps.
So these are some of the facts, but not necessarily the reasons why you are getting those measurements.
Please understand that the Internet speed tests measure the average throughput of a data package between their test server and your computer. As such it averages the combined data rates of the server, Internet link, your ISP's link to your modem, the modem passthrough, the router's interface, and your wireless network. It also doesn't necessarily measure all types of traffic between two end-points.
To best troubleshoot you would need to break down each of these links to determine the real bottlenecks. One thing we know is that the data throughput performance of your Express may not be sufficient enough for the bandwidth that your ISP is providing you.