The carpenter may simply know that turning the screw is imperceptibly instant in his experience, and that if the bookshelf isn't getting built quickly, the bottleneck is virtually never because he's waiting on screws to be turned. He's going to have to make a trip to the store anyway so he isn't concerned with whether turning the screw requires a tenth or a thousandth of a second.

He's more focused on the design and craftsmanship of the bookshelf, because he's built many excellent bookshelves to industry standard effectiveness in the past and measuring the number of screws he can turn in a second hasn't been relevant to the work.