Fallout 4 fps help

[Erthok]

So, I'm using my 4K tv to use steam big picture and play fallout 4 on. When I run the game at 1080p I get worse frame rates than I do at 2k. 2k is very playable at 62fps but the 1080p run is in the 20s. Does anyone know of a way to fix this? Would prefer the higher frames and I'm using the upscaler built into the tv. Please please please help, would like to run at 1080p and use the upscaler. Also to note the only resolution the launcher gives me is 800x400. So I have to set my resolution in the text document. Thanks! Also, 2k is at 30hz and the 1080p is 120hz

[l_zheng101]

What detail settings, and anti-aliasing are you using playing at 2k, 1080?  

[Erthok]

Ultra, everything maxed yet 2k performs better! Any idea?