You're right, with fab you only have three "speeds". These speeds are a function of your media and burner. I think Hun wants you to choose "slowest" in an effort to get down to 4X or perhaps 6X.
If this doesn't help, please post back with the MID (media code) and your burner (including current firmware).
Maybe an example will help. (Note the write speeds).
When I burn these TY's (TYG03) in a Liteon 1693s (KS0B) with express, they'll burn at 6X when "slowest" speed is chosen.