Why, when determining oxidants by iodometry, it is impossible to start titration immediately after adding potassium

Why, when determining oxidants by iodometry, it is impossible to start titration immediately after adding potassium iodide to the initial solution?

In iodometry, the rate of reaction between oxidants and iodide ions is insufficient, so titration is not started immediately after adding potassium iodide, but only after the time required for the reaction to complete.

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