Location: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SMS\DP
Name: RamDiskTFTPWindowSize
Type: REG_DWORD
The default value is 1 (1 data block fills the window) I used 8 which gave good speeds.
We can also tweak the TFTPBlockSize which has been around for many versions of Configuration Manager.
Location: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SMS\DP
Name: RamDiskTFTPBlockSize
Type: REG_DWORD
Value: <customized block size>
Test with values: 4096, 8192, 16384
Restart the WDS service after changing these settings.