Slow IIS 7.5 FTP Performance When Downloading

Hi, I have setup an IIS 7.5 FTP server on Windows Server 2008 R2 but when I download a file, the FTP server performance just dropping, the uploads go at 4 Mbits and I can see the FTP commands going very slow. I try FileZilla Server and I having the same issue but I fix it by changing socket buffer size. How I can change it in IIS? I really need help because when someone is downloading from the IIS FTP server, its slow for all other users and its unusable in that case. Thanks in advance for your help. Guillaume