High RPM on Idle Issue, Serpentile Belt Moving Too Fast on Idle, Eats Up a Lot of Gas

I have a 1994 Corolla LX Limited. Recently, I have noticed that the serpentine belt is moving too fast on idle. It seems that the RPM is too high. When I change the gear from neutral to drive, it becomes OK. During the traffic jam, my car eats up a lot of gas. A car mechanic has told me that the issue is associated with high RPM. But he cannot recommend me any solution.How can I solve the issue? via /r/cars http://ift.tt/2sNgnmA