Reducing the search area in an OCR process will cause a reduction of the memory required to complete the OCR and a great reduction in the amount of time that the OCR process takes. To reduce the search area you use the second overload of the Recognize method:

OcrEngine.Recognize(AtalaImage atalaImage, Rectangle bounds)

atalaImage is the image you are searching and bounds is the area you wish to restrict the search to (defined in pixels). bounds must be contained within the coordinates (0,0) and (atalaImage.Width, atalaImage.Height).

Original Article:
Q10245 - HOWTO: Reduce the Search Area of an OCR