So apparently the performance improvements to the Corvega Assembly Plant were meant for the console versions, and were snuck into the PC version. It seems the console versions were frequently running below 20 FPS in this area, and to increase performance they completely removed shadows inside the building... I'll need to go back and check it out myself. The first time I was there prior to the beta patch I never noticed any performance issues.
http://gearnuke.com/fallout-4s-late...formance-downgrading-visuals-pc-ps4-xbox-one/
hopefully it was a mistake for PC and Bethesda will fix it in the next patch