When saving plots to disk, you will want to make sure that the backend used is Agg. Otherwise, the plots can take a lot of time to be saved. To set the backend globally, first find your matplotlibrc file by doing the following:
import matplotlib
matplotlib.matplotlib_fname()
This will display the location of your currently used matplotlibrc file. Copy this file to $HOME/.config/matplotlib/matplotlibrc (matplotlibrc being a file not a directory) and ensure that backend is set as follows there:
backend : Agg