Hide geo from camera visibility = massive jump in rendertime

[TheImageFaculty]

I am rendering a shadow pass for a product shot.

I wish to cast the shadow of an object on a studio floor, so that the studio floor can be done in post. This means I will supply a beauty of the product and a shadow pass. It's been requested that the shadow pass includes the surfaces that are directly occluded by the product, (where the product and the surface it rests on touch). See the attached image of the alpha that my current setup creates.

I am happy with the result, but the render time is really poor when compared to the beauty. It's approx x12 times slower in this case. This jump in render time exists when I turn off camera visibility on the geometry of the product. This is the black shadows pass technique I'm using.

Please note, I realise that if I have the product 'cut-out' of the alpha via a diffuse/matte material or other method then render times are better. But as mentioned I want the occluded shadow and this is the only way I can see of achieving it.

Does anyone have any tip on how I can achieve the same result without the high rendertimes?

Thanks in advance,
Darren

Maya 21017 update 3
Octane plugin 3.06.2-116