I found a solution trawling the steam forums for my stuttering and slow down issues in full screen a while back I thought I'd share here as it may help some as it did me.

Go to C:\Program Files (x86)\Steam\steamapps\common\RiMS or where-ever your steam folder is located and locate the exe and right click go to properties and click the compatibility tab and tick Disable full-screen optimisations.

It's now smooth as a knife through butter for me now on full screen.