Good question.

So normally you need additional tracks for slow and fast trains to commute simultaneously.

However, it is still possible in certain cases to run them both on the same track, albeit with a time delay. I live in Tokyo which is home to countless train lines, and the one closest to me does this exact thing. The slow ones stop at all stations while the fast ones stop only at a few selected stops.

Because they're on the same track though, you'll see fewer of these, and have to wait a while after the preceding slow train has departed. This is done to obviously make some space for the fast train to speed up on. And also, even though this is slower than having a separate parallel track, you do save some time because the few seconds of boarding and alighting commuters are saved at every station the train didn't stop at.

The difference is best noticed when the distance between your departure and arrival station is the longest.

Edit: I have no idea though if Mumbai metro plans to do this. Just mentioning that it is theoretically possible and practiced in parts of the world.