If your INT is lower than a book’s nominal INT requirement, then each reading session with the book takes 1 turn longer to complete for each point of difference. However, a single reading session typically takes tens or hundreds of turns to complete, so a single-digit turn penalty for failing the requirement is almost meaningless.
This effect is hard to notice in normal play, because INT has considerable effects on reading time and experience gained per reading regardless. However, the warning for missing the requirement suggests that time costs explode once you dip under it, which really isn’t the case.