My light switch make a loud noise when used slowly; does it need to be replaced / fixed?
Yes, I would replace any switch that is consistently making the "popping" noise. This noise is from sparking of the switch contacts and is a bad thing and could lead to switch failure or even a fire. 

When choosing a new switch choose one that has a quality snap action to it. 

Lastly stop teasing the switches by moving them slowly. This is a bad practice and should be avoided. Operate the switches as they are meant to be used - snap on - snap off.