Here are a few questions you should try and answer in order to troubleshoot this:
1. Do you have any security product installed on the computers that might be scanning the traffic in real time and therefore limiting the speed? These are usually Internet Security type suites with firewall modules, or stand-alone firewalls. If the answer is yes, then disable them
2. What is the transfer rate when connecting the two computers together directly? (make one of the adapters an access point).
3. Is it only over wireless? Check transfer speeds for wired connections between the two computers, with the router in between.
4. Have you tried other file sharing protocols, such as FTP?
5. Have you tried switching the wireless connection channel? Don't go for 11, try different ones between 1 and 10. See if that changes anything.
6. What is the position of the antennas? Best position is like
here.
7. Do you have local peering (over the Internet) with anyone, which allows you to download with high speeds from them (over 2 MB/s)? That would be great for testing.