The Macho Brace excels growth, by making Effor Values double. It does make the user slower, however
When a Pokemon grows a level, so does it stats. When the Speed Stat elevates, while the user is holding a macho brace, the amount of speed is halved.
So in answer to your question, it will halve the speed when in battle, and when the user levels up, that will be halved too. But once the macho brace is removed, the stat will be fully restored