Rapid Reload is an upgrade for the Mortar Tower in BTD4 and BTD5 that makes it fire faster. It costs $170 on Easy, $200 on Medium, and $215 on Hard in BTD4. In BTD5 it is the first tier upgrade on Path 2, costing $210 on Easy, $250 on Medium, and $270 on Hard and $300 on Impoppable. In BTD6 it is the second tier upgrade on Path 2, costing $425 on Easy, $500 on Medium, and $540 on Hard and $600 on Impoppable.
In BTD6, Rapid Reload increases the attack speed of the Mortar Monkey by around +39%, for around +85% attack speed compared to a normal Mortar Monkey.