I'm working with the most recent R Studio IDE for Windows 10 with an i7 of 7th generation and 16G RAM. I'm experiencing severe lagging when working with an external 4K monitor (and without it as well to some extent) when working on a couple thousand lines long R Notebook files. Did you experience similar problems?
What version of RStudio are you using? We're working on a new version (1.2) with an upgraded rendering engine which should be faster on Windows 10. Would you be willing to try a daily build to see if it's faster for you?
I've tried using the latest one (RStudio 1.1.447 - Windows Vista/7/8/10) which has exactly the same problem as the one I was using previously. I also tried using one of the latest daily builds and the default resolution of any of those versions (I tried a couple) was very low - I couldn't switch it to my usual 4K resolution, which eventually made using R Studio impossible on my monitor. Not sure what the problem here was exactly.