Yes, you are right but this is to maximise the rakes use.
The lower speed is bcoz of high occupation of tracks for other super fast trains. And also the need to serve passengers on the intermediate stops between Ahmedabad - Vadodara.
For those who wish to travel fast, there are other super fast trains and for more faster -- the superb expressway - is available to cut the distance in almost 75 minutes as against superfast trains taking around 100 minutes.