NAS access and transfer speeds very slow from OS X but very good from Windows (Parallels)

Hi all,

I have a NAS (Buffalo Linkstation) which is connected to a TP-Link router (N750 - supports 802.11n up to 450 Mbps on 5Ghz) via gigabit LAN connection. From my iMac (2011) I can see the wireless link speed is 450 Mbit/s, but when I try to access the NAS I have 2 issues:

1) if I access a folder with many files (or subfolders) in it the speed becomes impossibly slow, I have a folder with about 3000 subfolders containing all my music and after 30 mins the ball is still spinning without showing the content of the folder. Basically my NAS is unusable.

2) if I transfer a single file (HD movie) from the NAS to the iMac the maximum speed is around 100 Mbit/s.

I have Parallels installed on the iMac with Windows 7, if I try the same things with Windows this is what I get:

1) the speed is very fast, all the folders are displayed immediately.

2) the maximum speed goes up to 200 Mbps, which I think is what I should get considering that I am not too close to the router.

It's very annoying that I need to use Windows to get good performances over my WiFi network on an iMac that I paid a lot of money... I need to use Windows sometimes but I didn't buy an iMac to use it with Windows..

I found some old posts about this but I could never find a good solution for this problem. I tried several suggestions but no one really helped, so I appreciate any help. I have Mountain Lion now but even with Lion it was exactly the same.

Thanks