TikZ/PGF may comprise a lot of heavy-compiling tikz/pgf plots generated e.g., via matlab2tikz.

For these cases, it is beneficial to compute tikz/pgf externally, i.e.:

#!latex

\usepackage{pgfplots} 
\usetikzlibrary{external} 
\tikzexternalize[mode=list and make, prefix=ext-tikz/]

See also #135

Thanks to Andreas Nautsch for this code.