Does decrease in voltage decreases the speed of electrons in a series circuit?
Neil's explanation is a good one but I thought I'd give just another way of looking at it. 

You were completely correct saying that the current is constant throughout the circuit. Say we have a 5V source and use wires to connect it to a resistor. 

I think where you may be getting confused is thinking all the energy the current contains is in its movement (kinetic energy) whereas it is almost all potential energy (you don't really notice it until connected to a location with a different potential) which is why there are a lot of references to gravity. Here is another image of the problem:

You are holding the marbles at the top of a hill - they have 5V of potential energy. You let them go onto this -almost- perfectly smooth ice which has a very small gradient - this is the electrons passing through the wire - they lose a little bit of potential energy. Then you get to a resistor - this is an extremely rough patch of grass but on a very steep bit of the hill. The marbles maintain their movement but it requires a loss of a lot of their potential energy moving against the grass. Finally there is another section of a small gradient of slightly rough ice of the wire the other side. Overall the marbles maintained their movement throughout but at the cost of different potential energies at different locations.

On a different note about how electrons actually act in the metals which I think you may find interesting. Without a voltage applied - electrons are flying about in both directions of the wire - at about a million km per hour - really fast but in both directions so there is no overall current flow! When a voltage is applied, they are still flying about extremely fast in both directions, but now there is also a small overall trend of movement (this is called the drift velocity). And when I say small, I mean only about a few meters per hour - really small! It is this mass movement we describe as the current flow.

Keep asking these sorts of questions though, it is good thinking!