First, i would revert render settings to default.
Then, make sure your curtain or any other objects in the scene do not have disabled shadow casting. That can break energy conservation and slow down a lot.
Then, try to remove curtain first, and test if scene renders in acceptable time. If it does, try to put curtain back, check material albedo, and make sure there are no fakes, as mentioned above.
If you want light to get through, then you will have to map opacity with some fine fabric pattern, or make the curtain just a little bit refractive with thin material mode enabled.