MDT WDS network boot is very slow

Server is running 2008R2 with WDS role enabled. Using MDT 2012 I have created a boot image and imported it into WDS.

Client computers can boot from the WDS and it says "Windows is loading files..." and is very slow loading the \boot\boot.sdi, often taking an hour or more.

The slowness only exhibits on the Toshiba laptop clients. Dell laptops have no issue and boot up within a few seconds. What could be causing this and how can I fix it?

I have imported up to date Atheros drivers for the Toshiba and optimized the boot image. I have also created a generic boot image with the same result.