You didn't say if you're running in 2.4GHz or 5GHz, and if 5GHz, whether you've wisely left it set to the default of using wide channels.
If you're running your AirPort Express in 2.4GHz, it will only use normal (narrow) 20MHz-wide channels so that it leaves room in the band for Bluetooth and other uses. So your maximum signaling rate is 130 or 144mbps depending on the guard interval in use.
The rule of thumb for overhead inherent in all 802.11 networks is that even in ideal conditions, your TCP throughput will only be about 50-60% of your 802.11 signaling rate. So it's possible under ideal conditions (completely clean channel, nothing else using your network, and your client is within a few meters of the AP) for your wireless link to match your broadband link.
If you have a Time Capsule on the same network, and it's configured to wirelessly extend the network, and your client happens to be joining the Time Capsule rather than the Express, and it's joining in 2.4GHz (because you have one of the old one-band-at-a-time Time Capsules from 2008, or the client happened to select 2.4GHz when it joined your simultaneous dual-band Time Capsule from 2009 or later), then yes, that will cut your bandwidth in half in the best case, and possibly by much more.
If you hold down option when clicking the AirPort menu extra, you'll be able to see the BSSID (the wireless MAC address) of actual AP you're connecting to, so you can determine which device you're really connecting to. You'll also be able to see what signaling rate you're getting, and what channel you're on.
For best results:
- Switch your network to 5GHz and use wide (40MHz) channels if you aren't already. Note that iPhones, iPod Touches, and PowerPC Macs can't do 5GHz with their built-in Wi-Fi radios. Even the iPhone 4 which does 802.11n doesn't do 5GHz. iPads (both 1 and 2) can do 802.11n in 5GHz. iPads don't take advantage of the fastest 802.11n modes, but it's better than 802.11a/g.
- Run an Ethernet cable between your Express and your Time Capsule. Use wired Ethernet for stationary devices. Save your wireless bandwidth for your mobile/portable devices.
- If you don't have a wired Ethernet backhaul between your APs, when you want the best Internet bandwidth, make sure you're associated to the upstream AP.