It's not the adapter's fault but Sony Vaio Pro's antennas or poor driver performance on Windows 8 since on my Lenovo G470 Windows 7 laptop with Wireless-N 7260 22.214.171.124 drivers, I get 81Mbps download and 39Mbps upload on my university campus Wi-Fi using speedtest.net
Also, you have to set the channel width on 2.4 GHz to "Auto" on the advanced adapter properties.
Follow this link for optimum performance: Wireless Networking; What are the recommended settings for 802.11n connectivity? ,
After removing McAfee I have 58 Mb/s download but closely to AP only. 10 m away of AP download speed is less then 1 Mb/s only. My Sony Vaio Pro 13 looses connection 12 m away of AP. I have Intel Dual band Wireless N 7260 card in SVP1321L1EBI notebook. I suffer weak connection in both bands 5 GHz and 2.4 GHz.
The scenario you put is clearly not due to drivers nor the adapter itself since I'm also running the Intel Dual Band Wireless-N 7260 on another non-Sony laptop. I get excellent range and throughput from both 2.4 and 5 GHz (5 GHz degrades faster though) from more than 15 meters. I'm 100% convinced that it's the way Sony placed the antennas in their Vaio Pro 13. Based on skimming through the thread, I saw someone claimed that the antennas were placed on the chassis of the laptop instead of being tucked in to the display bezel. That would clearly result to very poor range since the signal has to go through a lot of materials and interference with the metals inside the chassis would be much more induced. Hope Sony does a recall but this would cost them a lot.
I am sorry for the issue.
Leaving aside the fact that an internet speed test is not exactly a WLAN speed test, the best suggestion will be checking the suggestions posted by theveterans since you have latest drivers. You may also update the AP firmware, use the less crowded channel and good encryption settings like WPA2-AES.
If you already tried all of the above you may need to contact the system manufacturer for a possible integration issue.
I have the exact same problem with a Sony Vaio Pro 13. WiFi bandwidth is OK if I sit right next to the router but if I move out the bandwidth drops sharply. To quantify this I ran some download speed tests at different distances and my inexpensive Nexus 7 actually performs far better than the Vaio Pro 13 (see graph below). I labeled 35' as 'far' but it's actually really close by any standard.
I did contact Sony and their tech representative was not able to help. There is a thread on the the Sony Community and it has 538+ replies (as of this writing) since end of June so it's definitely a real issue for Sony. I bought this 1250$ laptop for my wife to connect to campus WiFi and now she's the only one in her class who can't get a connection.