Hollowing isnt very helpful to me because it does it in a way that all the brushes 'interlock' if you will and it takes longer to fix it than to make everything individually.
The most common cause of errors and increased r_speeds in new mappers' maps is bad carving. ie, they had snap to grid off/at 1, accidentaly had their carve brush stuck a few units into the ground, and it will carve a little one unit indentation, which is unnecessary, looks bad, and gives 5 more faces to render, and this happening multiple times in a map can cause lag.
It may seem very time consuming now to make each thing individually, but it will save time in the long run.