Does disabling automatic code completions help here? It's possible that RStudio's attempts to autocomplete file names, e.g. when typing within strings, is causing an overly expensive filesystem operation in your case:
I'm also curious whether (as you said in the original post) the slowness could indeed be attributed to attempts to list files on a networked drive. What is the output of, for example:
system.time(list.files("path/with/lots/of/files"))
system.time(list.dirs("path/with/lots/of/files"))
Do your R packages live on a networked filesystem? If so, you might also want to see how long it takes to query the set of installed packages:
system.time(installed.packages())
Hopefully this will get us closer to the underlying issue.