Daz finds it more difficult to calculate the light interaction if it passes through transparent or reflective surfaces.
the problem is that glass-shader are reflective surfaces
Yes, that's certainly part of the problem. One thing that you can consider if the window does have to be visible in the render is to delete the glass surfaces using the Geometry Editor so that the window panes are completely empty, rather than actually having glass in them. Or, at least, select the glass surfaces and set their opacity and glossiness all the way to zero. That cuts down on calculations a bit.
The other part of the problem is that iRay treats lights specially. If there's nothing between the light and the object being lit, iRay can do the lighting calculations very, very simply. As soon as the light has to pass through anything, things get much more complex for iRay because of the possibility that the material its going through will cause caustics, etc. (This is essentially what
mgomez0077 is saying.)
I don't see the need to put the light outside of the window if it isn't visible.
Completely true. We, as 3D renderers, get to ignore "reality" when it's convenient. Thus, you could easily move the spotlight just inside the window, set it to to be rectangular and the same dimensions as the window, and you'll have the illusion that light is coming through the window.
There is also one other thing you can try. If you look at the settings under Light > Area, there's one called "Light Portal" which defaults to "Off." If you create a spotlight just inside the window, align it to the window and set Light Portal to "On," what this is SUPPOSED to do is to tell iRay to pay special attention to this area of the scene because light will be coming from this area. In other words, this isn't a spotlight itself any more, but a hint as to where light is coming from. I fiddled with this some years ago and had mixed results with it in terms of reducing render times. In general, I moved on to other techniques, such as the ones
mgomez0077 mentioned with ghost lights, etc., but it's at least something you might want to experiment with as well.