The Macho Brace doesn't affect the growth of the speed stat EV wise, it only affects the actual stat. Similar to how the speed stat of paralyzed Pokemon is reduced to only 25%, the Macho Brace only halves the speed stat down to 50%.
EV wise, you'll still be getting increased Speed EVS depending on the Pokemon that you defeat
Tl;Dr: It halves the Speed stat, not the growth of it
Source: Experience EV training Typhlosion in Speed holding the Macho Brace