virtualenv code saves 2 minutes per build on our boxen.

gistfile1.sh

# Installing things takes a while. Skip if if nothing has changed.
# jenkins requires things to be html escaped to work properly, hence the weird command below
VENV_DIR=`md5sum requirements/* | md5sum 2>&1 | awk '{print $1}'`
if [ -e $VENV_DIR ]; then
  . /tmp/$VENV_DIR/bin/activate
else
  virtualenv --no-site-packages /tmp/$VENV_DIR
  . /tmp/$VENV_DIR/bin/activate
  pip install --download-cache /var/lib/jenkins/.pip_download_cache -r requirements/ci.txt
fi