How to make block a series of gulleys created by another block?

I can understand the concept of making a block of a solid positive object, e.g. a sphere, but having used an array of my block instance spheres to boolean difference semicircular gulleys from a solid, those resulting gulleys are slowing up my model, takes a while to save etc. 2 mins on the laptop, (not very practical !) 11 secs on new very powerful PC. (though 16Gb at present awaiting replacement mem stick pair of 16Gb)

Rhino always seems unable to cope with clones of a shape, despite latest PC , a series of same size holes, or rivets etc still causes delay and the feeling of slowness, if not also the screen freezing up and only a portion moving about as I rotate or pan. Making them blocks made those issues ok it seemed, but now the resulting gulleys are having the same effect though no screen issues as yet.

Can the resulting gulleys also be made as blocks seeing that they are now part of the solid the spheres cut into’ to the solid ? I cant sub-select a gulley, I get part of it.

On a similar note, if I have holes made by block rivets, can holes through say a 2mm walled object be made block to lighten the load on rhino ?

Any suggestions on how to get round this problem ?