PXE server is used only to boot the client systems. The imaging traffic does not go through the PXE service at all.
I'd suggest seeing if you can improve speed by removing hubs/switches etc from the path until you are able to locate the slow device. Other reasons for slow network speed would include incorrect NIC drivers, network loading, and network configuration.
You'll find more detailed explanations of network configuration elsewhere on the forum.
Some of these can get quite technical, and are probably best discussed with your network administrator.