The Ambient Occlusion is NOT calculated with lights, indirect illumination or maps:
It depends only on the proximity of the geometries.

So as logical consequence, if you want to get a quick AO just disable all these things. Watch the video:

Disable in “Global Switches“:

• Lights
• Reflections/Refractions/maps
• Just Active “Override Material”

Also set:

• Global Illumination OFF
• noise threshold to 0,01 (default value)
• Antialiasing to Adaptive Subdivision

Of course Noise Threshold and Antialiasing affect the definition.
You can use the settings you want: this one I’m suggesting you are really fast.