

It can slow things down, but once the light hits the curtain, the energy has gotten in, where it might dissipate slower, but it is still going to dissipate into the rest of the space.
Reflective curtains can do wonders, bouncing most of the light back out, mind. The advantage of reflective tint, though is that some of the absorbed light heats the window itself and passes that heat back out.
Shutters? Those’ll do it.