Summary: severe performance drop indoors with reflections enabled, even on low setting
Version: 17.2
Platform: PC
OS/Version: Windows 10 latest

Did you wipe old saves? yes
Did you start a new game? yes
Did you validate your files? yes
Are you using any mods? no
EAC on or off ? off

Status: old, but tested extensively since 17.2 experimental builds

Bug Description:
When you change reflection graphic setting from "off" to "low" performance degrades indoors from 110-120fps to 40-48fps immediately. A little less fps with "medium" and "high", but with these latest settings frametime spikes a lot more than low or off (tested with msi afterburner ingame graphs). Outdoors has also a performance penalty but just from 110-120 to ~90fps on same areas. The lighting is a bit different with reflections enabled, even without reflective surfaces around. Looks like the setting is enabling some light bouncing between blocks and surfaces that eats a ton of gpu resources, but that indoor scenes doesn't look arguably better compared with water reflections for example.

Screenshots before/after:

System: GTX 1080, I7 7700k, 16gb 3200mhz, SSD
2560x1440p, vsync off, exclusive fullscreen

Reproduce steps:
Switch reflection setting between off and low in settings while you're indoors at any building, big or small. I prefer to test outside cities and after cleaning zombies to avoid cpu spikes interferences. When you clic apply button to enable reflections the performance degrades immediately. To compare performance degradation between indoors and outdoors ,enable DM mode and fly in an out the buildings.

Actual result: more than 50% fps degradation indoors.

Expected result: performance degradation much less pronounced, like outdoors.