While experimenting with using dd under grub4dos to copy the contents of one large file to another large file on the same USB Flash drive, I found that if you use the built-in USB 2.0 driver in grub4dos 0.4.6a it is much quicker than just using the BIOSes driver (if using a USB 2.0 port).
e.g. Copying a 390MB file using the BIOS on an EeePC 904 took 104 seconds, but when the grub4dos USB driver was loaded it took just 23 seconds which was about the theoretical maximum for the USB Flash drive I was using (write speed was approx. 17MB/s max).
if you use the grub4dos 0.4.6 grldr file to replace your 0.4.5c grldr file, just add the command
to your menu.lst file, (or create a new menu entry for it), and see if it improves your boot times!
if you find any problems with incompatibility on some hardware platforms, please raise a bug report here. Note that USB 3.0 controllers are not supported by the 0.4.6 USB driver, so only use USB 2.0 ports.