Actually, at the bottom of page 15, the manual does state that holding down the [+/YES] and [-/NO] buttons provides continuous increasing or decreasing. [** EDIT **] I just realized that this is only for increasing or decreasing the values of the selected function parameter, not the function number itself -- You'll have to experiment to see if holding the Function button, along with holding down the [+/YES] or [-/NO] provides rapid function number change -- my guess is that it does not.]
Additionally, you can hold down the button for a relevant category that has a dedicated button on the panel, like dual voice, or split voice, for example. That will get you right to that specific function, and then if it's like the E400 series, you should only have to tap the Function button a few times from there to get to other parameters for that function.
For example, if you hold down the dual voice button, you'll get the voice number selection for dual voice. But then tapping the Function button from there should quickly get you to things like dual voice volume, dual voice octave, and dual voice chorus depth. And as Chris stated, if you go too far, the manual says that holding down the Function button while tapping the [-/NO] button will allow you to go back one step in the function list.
Hopefully, this helps with more easily navigating to different parts of the function list. Please let us know if you have any other questions!