It’s possible to change the size of the blocks used for the Trivial File Transfer Protocol (TFTP) transfer; the larger the block size, the greater the performance. Microsoft’s recommendation is to not set higher than 16384; in addition, you must use multiples of 4096, such as 8192, 16384, and so on. I got a huge performance boost from using a value of 16384. Perform the following steps:

Log on to the Configuration Manager PXE Windows Deployment Services (WDS) server.

Open the registry editor (regedit.exe).

Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SMS\DP.

From the Edit menu, select New, DWORD value.

Enter a name of RamDiskTFTPBlockSize and press Enter.

Double-click the new value.

Set the Base to Decimal and set the value (e.g., 16384).

Click OK.

Restart the WDS server service.

http://windowsitpro.com/configuration-manager/speed-sccm-tftp-performance-pxe-clients