Voynichese is a numeric cipher?

[eggyk]

The reason why I don't believe it is a codebook cipher (apart form my conviction that SPS=SBJ)  is common sense.  Encoding and decoding with such a cipher is very slow and tiresome, and I can't imagine why the Author would have chosen to do that.

People do things of monumental effort all the time, often for no reason other than simply wanting to do so. If the author enjoyed creating and writing in their cipher, it's no longer tiresome - it's just a hobby. 

I'm not sure why we treat 15th century human beings as purely rational creatures that would only do things in a utilitarian way. The exact opposite is normally the case. Why did people spend hours engraving their items, or carving animals from wood? Why decorate a plate that otherwise is fully functional, isn't that also slow and tiresome compared to doing nothing? 

Or taking a different angle, why do people climb tall buildings today, or swim across large open bodies of water? Often just to see whether they can. 

Genuinely, if your conviction is based on human beings always acting with common sense I think that is unwise. As my dad often tells me, common sense ain't that common.