mice() function does not run in parallel so you are effectively working with just one logical processor, take a look at the parlmice() function from the same package for parallelizing this process and taking advantage of all your cores.
https://www.rdocumentation.org/packages/mice/versions/3.4.0/topics/parlmice
Also, I think you are confusing R and RStudio, these are two different pieces of software, the former is the programming language and the later is an Integrated Development Environment. (This is nothing like traditional statistical software like SPSS, minitab, etc.) mice package is written in R but is not an RStudio product, that is why I have changed your topic category to general.