OSD: Downloading multiple files is very slow

I has a exe software. It is not a single file package, it contains a lot of files. when I installed this software in OSD, I found it would spend 15~20minutes to download files.

It was very strange. The software is only 750 mb size. the speed of downloading 6 GB OS image is also much faster than this software.

I know downloading multiple files is slower than downloading a single file, if their size are same.

But the downloading of multiple files in OSD is "really" very slow.

Why?  Cause of BITS or RDC?  How to fix it?

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PS：I update to SCCM 2012 R2 with CU1,  add the below into SCCM

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SMS\DP]
"RamDiskTFTPBlockSize"=dword:00004000

http://windowsdeployments.net/how-to-speed-up-pxe-boot-in-wds-and-sccm/

Now, the download speed up to 4 minutes, but it still slower than the normal copy & paste.