If its son of rome your talking about you shouldn't be dropping that low, 4 gigs of frame buffer should fix the low framerate. Sounds like a cpu bottleneck. There is one setting if cranked up to ultra that will tank most mid range cards, hbao if im not mistaken, set it to ssao and you should be fine.
You should be playing in full screen 1080p just fine probably seeing issues trying to play in that droped resolution. Try it at full resolution if it continues check your nivida control panel settings and ensure your drivers are up to date
Within Nivida control panel make sure power management is set to maximum performance will not harm anything but ensures your gpu isn’t being throttled by energy saving features