Effect of impurities on Enthalpy of vapourization
Adding impurities(which mostly have low heat capacity, like salt) decreases the enthalpy of vaporization. This is because the impurities lower the heat capacity of the solution as a whole, making the enthalpy of vaporization lesser than the original solvent. 

This is why even though the boiling point of a solution increases when impurities are added it is heated up faster(boils faster) than the one without impurities. For example, saltwater (salt is a non-volatile solute) boils faster than pure water because adding salt lowered the heat capacity of water as a result of which the enthalpy of vaporization is lesser.

Sources:


https://www.studocu.com/en/document/western-governors-university/natural-science-lab/mandatory-assignments/task-1-boiling-salt-water/5667142/view
https://www.thoughtco.com/adding-salt-to-boiling-water-607427


The addition of impurities lowers the heat capacity since the solvent molecules bind to the solute particles in a specific manner and hence require less heat to vaporize than they'd require when they're randomly moving.

Source:


https://socratic.org/questions/how-does-salt-change-the-specific-heat-capacity-of-water#143950