The actual throughput of a WLAN is never the advertised data rate of the underlying technology. For example, the top 802.11g data rate is 54 Mbps, but due to necessary overhead in the protocol, the actual throughput is at best only half as fast.
But the reality is that most WLANs are performing much worse than that! Why? Because a network comprised of more than a few access points is a complex environment for packet delivery, and non-optimized settings throughout your WLAN will drastically reduce performance.