Yeah, it is really weird. Hmmm, I'm just plugging it at the back USB ports. I tried switching ports and I they are the same. I just discovered something new (just now), when I disable USB 2.0 in BIOS, the slow boot speeds are identical with it Enabled. I disabled USB 2.0 in my other gigabyte motherboard (not G31m-eS2L) and now it is identical with the slow boot speed as my G31m-ES2L Enabled. I'm guessing that USB 2.0 is not working properly in the boot process.

However, If I'm inside the OSX (installed in the USB), its super fast. It's just the boot process.

BTW, thanks for confirming that you're not having this problem. I have to talk to Gigabyte regarding this issue.

EDIT:

Oh I forgot to mention that it is not DSDT related. I also have an external Windows 7 and XP USB installers. They all boot slow compared to a cd or dvd.