you're talking about painting plaster walls, right?
And as the first coat is using lots more, I assume it is bare plaster, not painted plaster?
IF SO, the correct procedure is to thin the first coat of emulsion on bare plaster (same applies to masonry paint on concrete or brickwork, which are also very aborbent) with water. This helps it soak into the absorbent substrate. The tin will say how much, it is usually about 10%. On very dry hungry concrete I have sometimes used 25% and applied two of these mist coats. If you were doing it yourself you would feel the wall sucking the moisture out of the brush. You can tell when you have applied enough to kill the suction, because following coats go on easier and faster because the paint stays fluid as you apply it. Any experienced painter will be familiar with this method of thinning the first, it is called a mist coat and will be recommended on the tin. It gives very little obliteration or coverage, so you still have to put two coats on top, but is very fast to apply and dry.
You say you are using a decorator, so it is up to you to find a good one by asking friends and neighbours for recommendations, and looking at their houses to see what you think of the quality of work, and asking what they were charged. If you were DIYing I would recommend that you applied a couple of coats of Dulux Trade Supermatt Brilliant white (this is not a vinyl paint so has better adhesion for future coats) to the bare plaster. This gives a smooth flat surface of uniform absorbency, which is very easy to paint on. It also highlights any remaining blemishes on the surface to the eye, so you can fill, scrape or sand them smooth before patch-priming with white again to give you as near a perfect finish as you can manage. Then when you come to apply your colour, you will use less, and will have a uniform colour and texture with no patchiness. However a professional will not want to do this (except in his own house) because it is labour-intensive, and time is money, and most customers won't appreciate the work that goes into achieveing perfection
and are unwilling to pay for it.
But bear it in mind if you ever want to don the overalls yourself.