You can burn no slower than what your burner will offer, and it would be wise to avoid trying to do so [for quality’s sake]. All you can do is avoid using your computer while it’s burning.
Besides, burning slower usually doesn’t make better discs, unless you run into buffer under runs frequently. And even then, if your drive has buffer under run protection, it will probably not impact write quality significantly.
What is your setup like, specification-wise?