None have answered this quite right yet.

Alright,

Macho Brace doubles the amount of Effort Points you recieve from the opponent's pokemon, saving you time in maxing out your Effort Points by reducing the number of battles you have to fight. The disadvantage is, that it reduces your speed by half while you're wearing it, so the speed reduction is NOT PERMANENT.

This explains why Muffin Man's Breloom can't keep up and Carbonite4's Sceptile can't take vitamins, his Effort Points are likely to be maxed or close to being maxed.