These days it is not necessary to have gem documentation installed locally. This unnecessarily increases installation time. Unless you have specific requirement for locally available documentation, you can skip the installation of rdoc and ri documentation when installing gems.
Just add this to your home directory in
gem: --no-document # skip installation of all documentation or gem: --no-rdoc --no-ri # For some reason the above configuration did not work for me on my office computer # Or add these to skip either ri or rdoc documentation # gem: --no-ri # gem: --no-rdoc
With this file in place, next time you install a gem, no documentation will be installed.