The speed of TFTP transfer is determined by the TFTP blocksize that can be successfully transmitted over the network. The smaller the blocksize the slower the transfer. Therefore, the largest blocksize that can be successfully transmitted will be fastest.
The bootdisk determines the settings for TFTP transfer.
By Default the blocksize is 1456 bytes.
This block size is chosen to fit within a standard MTU in case the boot-server is remote and served by IP forwarders. It is possible to increase the block size for a LAN connected boot-server up to a maximum size of the connected LAN segment. If the packets are routed it is possible for fragmented UDP packets to be dropped or delivered out of order and TFTP's recovery mechanism may not cope with this leading to errors.
Most LAN based networks are capable of handling a blocksize of 65536 bytes without a problem.
Edit the mage Prepare System template for WinPE based Bootimages to increase the default blocksize and create a new Boot Image with this template.