IDL, like the word means, Indirect Light.
But indirect Light, is also "light".
it is just not coming from a "light".

it gives a lot more options to settle the shadows, the interaction between objects and their natural reflections.

There is only "One sun". And by day, I do not put on extra lights...(not outdoors anyway ;-) )

It requires some serieous "rethinking" about the use of light, and shadows.

For slower PC's, keep the settings low, and use the "minimum needed".
Like modern logistics work, delivery "just in time".
"Ligth" , just in time.
For outdoors, and certainly with BB's great Env sfere, that mostly means only one light.
And even that one, I often turn to lower intensity.
But the result is, that you need to play more with the softer shadow settings.
There are no "hard" shadows in real life.
Turn on a light in a room.
Where does the light come from? Look around you?
How does the rest of the room get its light? The floor beneath the table? it is not black I hope?

Start with the lower render settings, and work your way up.
Corrections can be made "on the go", and you'l save yourself a lot of time.

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!