Beyond duty and tax, budget for terminal handling charges, storage once any free time expires, demurrage (a per-day charge for keeping a shipping line's container past its free period) and detention (a similar charge for keeping the physical container itself too long), inland trucking to your warehouse, and the fees charged by your broker and freight forwarder for their services.
These charges are often small individually but add up, and because several of them are time-based, a delay anywhere in the clearance process directly costs you money. Planning clearance to happen quickly is a cost-control measure, not just a convenience.