Windows PE Based plugins takes a lot of time to load within a virtual environment.
This issue is caused by "gpxelinux.0" rom file.
Change your DHCP rule from "gpxelinux.0" to "pxelinux.0" to solve this issue.
Linux DHCPd:
filename "pxelinux.0";
Windows DHCP Server:
Rule 67 "pxelinux.0"
Thanx goes to w4rh0und for finding the solution.