Rapid Fire

Rapid Fire cuts the cooldown of every Monkey Ace attack by 25%, which works out to roughly a third more attacks per second. The basic eight-dart volley goes from one every 1.68 seconds to one every 1.26 seconds, and if you later buy Exploding Pineapple the pineapple drops speed up from every 1.6 seconds to every 1.2 seconds. Because it touches all attacks, it also speeds up bombing runs and any other weapon the plane gains later.

When to buy it

On its own, Rapid Fire is not a great deal. A second plain Ace gives more damage for the money than one Ace with this upgrade. Its real value is as a crosspath. It is the standard top-path pick for Bomber Ace and Ground Zero builds (1-3-0, 1-4-0), where the faster bombing runs make the bombs land more often and more consistently. Neva-Miss Targeting and Spectre also like it, usually together with Lots More Darts as a 2-0-3 or 2-0-4.

Buy it when you are already committed to one of those paths, not as an early-round damage upgrade. It also unlocks Lots More Darts, the next step toward Fighter Plane.

Is it worth it?

As a crosspath, yes: $450 for a third more attack speed on everything is cheap once the Ace is carrying real upgrades. As a standalone early purchase, you are better off saving for a stronger tower or a second Ace.