On Mojave, I notice it less in RStudio than I do in the plain R GUI. The underlying issue, I think, is that R just doesn't have very good garbage collection, and after a long session in R, I notice that my free RAM is down to the millions of megabytes. That's when it's time to quit, use a utility to free memory, and restart. Because RStudio runs on top of R, I'd expect long, heavy usage to show the same behavior. But I'm a mere user, not an engineer, and I could be wrong about this, as about so much else.