I was having an issue with very slow download of the WinPE WIM image, the screen shows ‘Windows is loading files …’ but it takes between 7 minutes and 1 hour to load the 150mb WIM file over a 100mb link.
After spending a lot of time looking into this, and changing a lot of settings including WDS TFTP block (65536) and window size (64), the RamDiskTFTPBlockSize setting in HKEY_LOCAL_MACHINE\SOFTWARE\wow6432node\Microsoft\SMS\PXE (http://sccm.haas.se/?p=15), the final solution involved settings on the Intel teaming settings on the PXE server:
I needed to leave Jumbo Packets disabled as the client couldn’t handle it and it failed. I increased Transmits and Receive Buffers to maximum values of 2048 and performance was dramatically increased.
Once this was changed, the WIM download was well under 1 minute!