Principally, the decimate modifier reduces face count. A smaller geometry leads to less memory usage and a shorter render time.
BUT: The modifier is slow. Really slow (Don't blame it, reducing geometry isn't a simple task). Before rendering, all modifiers are "applied" so that they can get rendered in the "Syncronizing object" phase as Jerryno said. This will add some overhead to your rendering time.
If you can afford it, apply the modifier. Then it will calculate once and for all instead of at every render. Eventually back up your object somewhere else just to be sure.
Also please keep in mind that the decimate modifier is a "I messed up the polygon count" quick fix. If you need it, in most of the cases that's a hint that you made a mistake somewhere. See here for alternative solutions.