Does impurities cause a substance to have no fixed boiling points? Does it also cause substances to take a longer time to boil?

Maybe. Depends on the nature of the impurity.
... If the impurity is a liquid with a lower boiling point than the original sub. then the mixture will have a LOWER boiling point. If the impurity is a solid, then it will cause the mix to have a higher boiling point.
--- Longer time to boil .. if BP is higher then yes, it will take longer to boil.