Hi,
It seems you can only use regex to specify the search term, but not the folder. One option you could consider is using the terminal window in RStudio and using grep.
grep -iRl "data.frame" --exclude-dir=.Rproj.user
In this case, all files containing the term data.frame will be returned, apart from those in the folder ./.Rproj.user/ (a hidden folder).
It's not perfect, but it might help speed up things.
PJ