Our good friend Heinrich wrote a great article back in 2014. https://thedesktopteam.com/heinrich/sccm-2012-boost-pxe-deployments-speed/
Recently I have been tasked to deploy some Surface Pro 5 devices and the PXE Boot image was taking just over 7 minutes to download. My goal was to reduce it as much as possible, but at the same time keep the integrity of the PXE Boot process.
I found this fantastic post by
Jörgen Nilsson http://ccmexec.com/2016/09/tweaking-pxe-boot-times-in-configuration-manager-1606/ it includes a table with his tests and the resulting speeds.
In my case with multiple device models including the Surface Pro 5 I decided to go for 8 as my value for the key bellow. The 7minute download became a 1min 37sec download.
The default value is 1 (1 data block fills the window)
I was unable to use the RamDiskTFTPBlockSize registry key, as setting any values to it made the pxe boot fail.
Do not forget to restart the WDS service after you change the registry settings.