\documentclass [12pt] {article}
\usepackage{tikz}
\usetikzlibrary{hobby,external}
\tikzexternalize
\begin{document}
\begin{tikzpicture}[Hobby externalise]
\draw (0,0) [curve through={(1,1)}, save Hobby path=path]to (2,2);
\draw [xshift=1cm] (0,0) [restore and use Hobby path={path}{}];
\end{tikzpicture}
\end{document}