You can easily determine this with a stack of blanks (all the same). My cd writer is a 52x.. I made an image of random stuff 700 MB in size.. then disk at once'd it at 52x, 48x, 40x, 32x and because I felt like it, I kept on going to lower speeds.
Then I did a Kprobe scan of the burned images I did, the best quality scans was at 40x. Lower was not better.
Therefore for that media (Fujifilm TY) 40x was the sweet spot for my burner.
You have to do the same test on whatever media/drive you're using. And the sweet spot usually isn't the highest speed or the lowest the drive can support.