During a pxe boot, when the boot image file is being loaded in the client, it should not take any longer than a few minutes time depending on the size of the boot.wim and your network. If it seems that your PXE boot times are extremely slow, you may be able to speed up the process by increasing the TFTP block size. This article will show you how to speed up PXE boot in wds and SCCM.
Trivial File Transfer Protocol (TFTP) is the network protocol used for downloading all files during network boots. TFTP is an inherently slow protocol because it requires one ACK (acknowledgment) packet for each block of data that is sent. The server will not send the next block in the sequence until the ACK packet for the previous block is received. As a result, on a slow network, the round-trip time can be very long.
change the Maximum Block Size to 16784 on your WDS server