@garcle - see my comments above in post #6. The way to test this would be to go into convert, choose search & replace and click one of the wizard buttons. That will ask Calibre to convert the document in the exact same way that my ISBN extract does. Check how long it takes for it to do this with your big PDF file to get to a point of text being displayed in the wizard box, versus how long it takes the extract ISBN functionality.
If the times are comparative, there is nothing I can do, at least not without rewriting the text conversion functionality to perhaps say just convert a small % of the document. Which I have no intention of doing myself
OTOH if you think the ISBN functionality is still significantly slower than the S&R wizard then I could take a look at it. If you point me at a download somewhere of a PDF typical of the issue I will see what I can do.