Compartmentalization is a perk available to all heroes at the start of the game. Compartmentalization makes Lightweight Equipment Mastery free to take immediately upon the start of the game, without spending a mastery point. (Compartmentalization does not actually activate Lightweight Equipment Mastery; it just removes its Mastery Point cost.)
Compartmentalization can be effectively paired with the Mercenary perk, as the initial minerals given by Mercenary cover the majority of the over-time costs of Lightweight Equipment Mastery, and the extra skill points give extra chances to get talents like Supercharge for the energy and regeneration to capitalize on the reduced cooldowns.
While Compartmentalization seems like a good idea on paper, in practice there are often more effective perks. The bonuses provided by Lightweight Equipment Mastery are not very relevant in the first 10-20 levels of the game, as heroes do not have the energy capacity or regeneration required to keep up with the reduced cooldowns.