Recently ordered one of these for use on a Raspberry Pi. Just for giggles I plugged it into my main laptop - Lenovo ThinkPad T530 w/ built-in Intel b/g/n wifi, running Ubuntu 14.04LTS. Basically I wanted to confirm it worked with Linux in general before I tried fighting with it on the RPi.

The TP-Link 822N showed up no problem in the network configuration manager... but I thought it a bit odd that this brandy new "300Mbps High Gain Wireless USB Adapter" was showing up as only 18Mbps, compared to the internal wifi adapter that shows up as 54Mbps.

Am I expecting too much from a USB wifi adapter, that it should be as fast as the internal wifi, much less the advertised speed?

Here comes the tricky part of the story because you basically have two bottlenecks in your new network adapter. The 18 megabytes sound about right for a USB version 2. While on the other hand 18 megabytes could also be the current network performance experienced at that moment. You could switch your network adapter from USB port making sure to plug it in a 3.0 USB. See if the speed changes. If not, it might be due to your current network performance.

Now let's talk about the other part of the trick. WiFi network adapters speed is a lengthy topic and people often get frustrated at results. I would suggest you to search for WiFi network installation and configuration manual.

Wifi can be extremely tricky to deal with. Yes you can have them performing at what the label on the box says under certain conditions but that's not a given. Basically to set highest speed and access points can perform at is the highest speed of your lowest network device connected at the moment . So if you buy 5 network cards that perform at 300 and one that performs at 54, you're basically downgrading your whole access point and connected devices to work at 54. So in order to truly check the speed of a network device, it needs to be hooked or connected on its own making sure the access point its not connected to any other device. That will make sure both ends are truly talking at the same speed.

I regularly connect network devices at my customers and it's not where they complain about speed of the devices. There are many other reasons why a wifi won't perform at top speed: distance, humidity, physical layout, interference just to name few.

So for starters (and this is a little hint guide for your config), make sure the network adapter and the access point on the only once connected against each other. Then on your access point try the lowest speed first and measure the speed with a regular FTP transfer. write down and then repeat the same exercise going into B, G or N configurations. Lastly, configure your sxs network and your network card to use the most satisfactory configuration you wrote down. Now, do the same exercise for each network device and compare the speeds. Make sure those configurations are compatible across the board. You're a new network speed will be the lowest speed of your lowest device. Good luck!