Externalization of TikZ images and PGF plots is often very useful to speed up the latex compilation of a document. Hereby all images within the document are only compiled once. And, if not changed, only their PDF output is included when the document is compiled the next time.

To enable externalization one simply has to include the TikZ external library. See section 53 in the PGF/TikZ manual on TikZ picture externalization.

\usetikzlibrary{external}
\tikzexternalize[prefix=MYPATH/]    % defines a folder for intermediate outputs

And you also have to create an empty folder named MYPATH. However, to make it work, your latex compiler has to be called with -shell-escape to be able to spawn new processes.

pdflatex -shell-escape

Unfortunately it is not possible to set this option in the overleaf menu (at least for the version I’m working with). But I found this on stackexchange with a link to an overleaf blog post about a latexmkrc file that allows to set some environment parameters. Therefore one simply has to create a new file in the overleaf project root named latexmkrc and add the following to it:

$pdflatex = 'pdflatex -shell-escape';

When I did this I first got the following error message:

I can't write on file `MYPATH/output-figure0.md5'.

<to be read again> 
                   \relax 
l.766 \end{tikzpicture}
                       
(Press Enter to retry, or Control-D to exit; default file extension is `.tex')
Please type another output file name
! Emergency stop.
<to be read again> 
                   \relax 
l.766 \end{tikzpicture}

I figured it maybe is just some file rights issue, so I created an empty file named output-figure0.md5 in the MYPATH folder. Et voila – this worked for me! From now on overleaf stores the individually compiled images and only recompiles them if their code changes.

Yet, I was not able to get access to these intermediate outputs in the GUI as overleaf apparently stores them in ...project_id/outputs/MYPATH/ although I was hoping they are stored in ...project_id/MYPATH/ (you can see it in the logs). But yeah, it works and I don’t really need access to these files anyway…

Update: The intermediate outputs can be accessed via “Logs and output files” and then on the bottom right “Other logs and files”.