Different PDF editors OCR PDF files at different speeds. Today at work I had a chance to use FoxIt Phantom for the first time. It includes at 'Quick OCR' option. I tried it and it was fast and relatively accurate, but not so impressive compared with its competitors. The FoxIt Quick OCR tool took 1 minute and 10 seconds to create a searchable version of 39 page chart. While the resulting quality wasn't bad, Nuance Power PDF Advanced took only 36 seconds for the same PDF, and actually rendered noticeably superior OCR text.
Adobe Acrobat -took 2 minutes 9 seconds on the 'Searchable Image' setting, and 2 minutes, 21 seconds using the superior ClearScan setting.
Not a very scientific study, but it appears as though Nuance has a clear edge.