Does Nero recode2 compile a DVD faster on hard disk or dvd rom?

I am backing up a DVD movie using NERO recode2. Now, It 's taking me 50 minutes to compile if I copy the whole DVD movie to hard disk first. While it only took me 20 minutes if I compiled directly from my DVD burner.
This really confuse me because I always thought that it would be faster if reading from hard disk than DVD burner
My computer:
p4 2.4g
512M ram
winXP SP2
160g IDE harddrive.
Liteon 411s burner.

Thanks for Anyhelp.

A few more details may help. Is this a copy protectected DVD? Are you shrinking it in Recode?