What are the limiting factors that prevent access through WiFi to achieve the highest speed the radio link can provide?
Assuming, there is only 1 device (the wireless one) on the network, it's sitting right in front of the router.
Cable broadband Internet connections in some countries go up to 100 Mbps.
802.11n WiFi is capable of 600 Mbps.
But your particular WiFi router might not provide that speed. Older WiFi devices might operate at 54 Mbps or lower.
In both cases, you won't see 100 Mbps of data because some proportion is consumed in low-level packet overheads and background chatter between devices.
There can be interference on one or more bands from other nearby WiFi devices (your neighbour), from non-WiFi devices that use the same frequencies and from poorly shielded appliances.
Make sure your channel is not used by another WiFi network, since this reduces the signal to noise ratio and limits traffic. You can check channels with an WiFi-Analyzer smart phone app for Android.
Also make sure both: router and wifi adapter support the same standard e.g. 802.11n.
And last but not least, if you use an unstable driver for your adapter you might also not end up with maximal speed.
If you plug in your adapter via USB, make sure USB 2.0 or higher is used.
Also, if you are using 2.4GHz, there are a lot of other devices that share the same radio waves.
It can cause the Wi-Fi to resend packets when it gets congested.