In low light use only the central focussing zone. It's a cross type, more sensitive than the others. If you're using dynamic focussing zone choice the camera is losing time trying to figure out which zone is in focus.

The 35mm f1.8 is my first choice for an available light lens because it's a "normal" view and I can use it in lots of situations. With stage performances, sitting in the front row I can get a good view of the scene with the performers from head to toe. I also have two relatively inexpensive lenses (which I bought second-hand):

- the 50mm f1.8
- the 85mm f1.8

I particularly like the view of the 85mm, which takes me right into the scene with a good view of the performers faces

Without flash probably need 6400ISO. You can use the 35mm and crop but the look of the image is getting a little mushy at this speed due to noise reduction.