I have experimented a fair bit. The quickest way to paint en-masse is to undercoat white or grey and then use washes of various colours. This acts both to cover the figure and to provide shading, as the wash (and pigment) get more pooled in the recesses. Moreover, it is easier to spread the wash than paint over a figure. On the other hand, the colours are a bit duller, which is probably realistic, but not so great to my eye.
The second fastest method is to use a dip (like Army Painter's or a homemade one). To do this you undercoat white or grey, block the colours in and either dip the figure in the tin or paint it on. It gives great shading and has a great advantage of being extremely durable! However, the disadvantage is that it makes your bright colours look dirty. You can of course brighten these up again, but that loses valuable time. I find that as long as the bright areas are not large it doesn't really matter in 15mm.
DBA armies tend to be small and if you want to give them the best treatment then paint them as you would larger figures - i.e. undercoat, base coat, wash and highlights. The most important area for washes is the skin and light colours (clothing). I find that washes on colours like (medium) blue, green, red, etc. don't give you as much incremental improvement and tend not to do them in 15mm, except for leaders and individual models.
Having said all that, if you want your figures fairly bright and with lots of contrast (which in my view is the most suitable for 15mm and smaller scales) then paint using the "Dallimore" method - undercoat with black, paint a dark base coat, lighter mid coat and even lighter highlights. In principle you could use washes here, but that would be overkill - you shouldn't need them. Once you get used to it, this method is also fairly fast.
One last thing - painting horses. This is one of the areas where I find the washes most useful. You can knock out many horses and very quickly by using an appropriate basecoat (white, grey, sandy, various shades of browns) and applying a selection of washes and/or inks to get the tone you want.