As bunkmate74916 says, turning off shadows is one solution.
My personal solution is to use Christmas lights. I turn off all the lights in the base, and string Christmas lights throughout the whole building to light everything up. Christmas lights have a surprising amount of brightness, and don't cast any shadows. I now have my shadows set to max and don't have any FPS troubles in the fully lit base anymore, so I can still enjoy the visual of the sun shining through the windows without playing a slideshow. They don't go out in a power outage, though whether that's an up side or a down side depends on whether you like the tension of the base suddenly going pitch black without warning. You also miss out on the possibility of being jumpscared by your own shadow, but some things must be sacrificed on the altar of smooth FPS.
Perhaps the best part is they are relatively cheap, because you can buy one and just keep stringing the same set of lights throughout the whole base and even the yard if it pleases you. I hope this helps.