Introduction

Hello, I am facing very long compile times of more than 14 minutes for a ~130 page document, including around 40 PGF-Plots/TikZ-Images. Due to LaTeX's memory limits and (in fact) for speed up reasons, I decided to use the external-package.

Then I noticed, that each image takes up to 25 seconds to render (looking into my pdflatex.log from Miktex 2.9.
Edit: When compiling the same TikZ-images using TikzEdt, it takes only a split second from beginning to the end of compilation and the PDF being there.

Today, I created a MWE for demonstrating this effect, which seems to increase with the number of other packages loaded:

MWE (find it also here: Github)

documentclass[]{article}



usepackage{pgfplots}

usepackage{graphicx} 



% Comment out the next three lines for faster compilation time

usepgfplotslibrary{external}

tikzexternalize

tikzsetexternalprefix{figtemp/}



begin{document}



section{First}

    begin{tikzpicture}

        tikz draw[thick,rounded corners=8pt]

        (0,0) -- (0,2) -- (1,3.25) -- (2,2) -- (2,0) -- (0,2) -- (2,2) -- (0,0) -- (2,0);

    end{tikzpicture}



section{Second}

    begin{tikzpicture}

        draw (-1.5,0) -- (1.5,0);

        draw (0,-1.5) -- (0,1.5);

    end{tikzpicture}



section{Third}

    begin{tikzpicture}

        draw (0,0) arc (0:315:1.75cm and 1cm);

    end{tikzpicture}



end{document}

This example gives me the two respective pdflatex.log-Files, depending on whether externalization is enabled or disabled:

Externalization enabled

Total time: ~14.0 s

2019-03-05 13:21:31,240+0100 INFO  pdflatex - starting with command line: pdflatex.exe -synctex=1 -interaction=nonstopmode -enable-write18 TeX-TikZ-slow-MWE.tex

2019-03-05 13:21:31,246+0100 INFO  pdflatex - allowing known shell commands

2019-03-05 13:21:31,246+0100 INFO  pdflatex - enabling input (output) from (to) processes

2019-03-05 13:21:31,246+0100 INFO  pdflatex - allowing all shell commands

2019-03-05 13:21:33,607+0100 INFO  pdflatex - executing write18 shell command: pdflatex -enable-write18 -halt-on-error -interaction=batchmode -jobname "figtemp/TeX-TikZ-slow-MWE-figure0" "deftikzexternalrealjob{TeX-TikZ-slow-MWE}input{TeX-TikZ-slow-MWE}"

2019-03-05 13:21:33,885+0100 INFO  pdflatex - starting with command line: pdflatex -enable-write18 -halt-on-error -interaction=batchmode -jobname figtemp/TeX-TikZ-slow-MWE-figure0 deftikzexternalrealjob{TeX-TikZ-slow-MWE}input{TeX-TikZ-slow-MWE}

2019-03-05 13:21:33,891+0100 INFO  pdflatex - allowing known shell commands

2019-03-05 13:21:33,891+0100 INFO  pdflatex - enabling input (output) from (to) processes

2019-03-05 13:21:33,891+0100 INFO  pdflatex - allowing all shell commands

2019-03-05 13:21:36,177+0100 INFO  pdflatex - finishing with exit code 0

2019-03-05 13:21:36,326+0100 INFO  pdflatex - executing write18 shell command: pdflatex -enable-write18 -halt-on-error -interaction=batchmode -jobname "figtemp/TeX-TikZ-slow-MWE-figure1" "deftikzexternalrealjob{TeX-TikZ-slow-MWE}input{TeX-TikZ-slow-MWE}"

2019-03-05 13:21:36,614+0100 INFO  pdflatex - starting with command line: pdflatex -enable-write18 -halt-on-error -interaction=batchmode -jobname figtemp/TeX-TikZ-slow-MWE-figure1 deftikzexternalrealjob{TeX-TikZ-slow-MWE}input{TeX-TikZ-slow-MWE}

2019-03-05 13:21:36,619+0100 INFO  pdflatex - allowing known shell commands

2019-03-05 13:21:36,620+0100 INFO  pdflatex - enabling input (output) from (to) processes

2019-03-05 13:21:36,620+0100 INFO  pdflatex - allowing all shell commands

2019-03-05 13:21:39,167+0100 INFO  pdflatex - finishing with exit code 0

2019-03-05 13:21:39,311+0100 INFO  pdflatex - executing write18 shell command: pdflatex -enable-write18 -halt-on-error -interaction=batchmode -jobname "figtemp/TeX-TikZ-slow-MWE-figure2" "deftikzexternalrealjob{TeX-TikZ-slow-MWE}input{TeX-TikZ-slow-MWE}"

2019-03-05 13:21:39,583+0100 INFO  pdflatex - starting with command line: pdflatex -enable-write18 -halt-on-error -interaction=batchmode -jobname figtemp/TeX-TikZ-slow-MWE-figure2 deftikzexternalrealjob{TeX-TikZ-slow-MWE}input{TeX-TikZ-slow-MWE}

2019-03-05 13:21:39,592+0100 INFO  pdflatex - allowing known shell commands

2019-03-05 13:21:39,592+0100 INFO  pdflatex - enabling input (output) from (to) processes

2019-03-05 13:21:39,592+0100 INFO  pdflatex - allowing all shell commands

2019-03-05 13:21:41,817+0100 INFO  pdflatex - finishing with exit code 0

2019-03-05 13:21:42,029+0100 INFO  pdflatex - finishing with exit code 0

2019-03-05 13:21:42,485+0100 INFO  pdflatex - starting with command line: pdflatex.exe -synctex=1 -interaction=nonstopmode -enable-write18 TeX-TikZ-slow-MWE.tex

2019-03-05 13:21:42,489+0100 INFO  pdflatex - allowing known shell commands

2019-03-05 13:21:42,489+0100 INFO  pdflatex - enabling input (output) from (to) processes

2019-03-05 13:21:42,489+0100 INFO  pdflatex - allowing all shell commands

2019-03-05 13:21:45,223+0100 INFO  pdflatex - finishing with exit code 0

second run, using the externalized images

Total time: ~2.4 s

2019-03-05 14:15:03,628+0100 INFO  pdflatex - starting with command line: pdflatex.exe -synctex=1 -interaction=nonstopmode -enable-write18 TeX-TikZ-slow-MWE.tex

2019-03-05 14:15:03,629+0100 INFO  pdflatex - allowing known shell commands

2019-03-05 14:15:03,629+0100 INFO  pdflatex - enabling input (output) from (to) processes

2019-03-05 14:15:03,629+0100 INFO  pdflatex - allowing all shell commands

2019-03-05 14:15:06,080+0100 INFO  pdflatex - finishing with exit code 0

Externalization disabled

Total time: ~5.1 s

2019-03-05 13:23:19,734+0100 INFO  pdflatex - starting with command line: pdflatex.exe -synctex=1 -interaction=nonstopmode -enable-write18 TeX-TikZ-slow-MWE.tex

2019-03-05 13:23:19,742+0100 INFO  pdflatex - allowing known shell commands

2019-03-05 13:23:19,742+0100 INFO  pdflatex - enabling input (output) from (to) processes

2019-03-05 13:23:19,742+0100 INFO  pdflatex - allowing all shell commands

2019-03-05 13:23:22,045+0100 INFO  pdflatex - finishing with exit code 0

2019-03-05 13:23:22,476+0100 INFO  pdflatex - starting with command line: pdflatex.exe -synctex=1 -interaction=nonstopmode -enable-write18 TeX-TikZ-slow-MWE.tex

2019-03-05 13:23:22,485+0100 INFO  pdflatex - allowing known shell commands

2019-03-05 13:23:22,485+0100 INFO  pdflatex - enabling input (output) from (to) processes

2019-03-05 13:23:22,485+0100 INFO  pdflatex - allowing all shell commands

2019-03-05 13:23:24,842+0100 INFO  pdflatex - finishing with exit code 0

Conclusion

Externalizing my three (very simple) TikZ-Images blows up compilation time almost by a factor of 3 in the first run.

When using a tool like TikzEdt, all my TikZ-images compile in a split second.

What can I do, to speed this up?

Edit:

I found out that TikzEdt uses a precompiled .fmt file, what makes it so fast. As I've got no experience in precompiling, I could use some help for this and how to pass it to the tikzexternalize calls of pdflatex. In this question someone is tryin the same, but I can't seem to organize my preamble to work as desired and meanwhile speed up the externalized image compilation.

tikzexternalize will always take longer than compiling without externalization on the first run. It needs to create the figures just like it has to do w/o tikzexternalize, and on top of the usual work, it has to export them to files. This includes overhead that is added by calling new instances of pdflatex and interacting with the OS separately for each picture.

The performance gain is best illustrated with a slightly more intensive TikZ example, as suggested by @Phelype Oleinik:

documentclass{article}

usepackage{pgfplots}

usepackage{graphicx} 

usepgfplotslibrary{external}

tikzexternalize

tikzsetexternalprefix{figtemp/}

begin{document}

section{First}

        begin{tikzpicture}

                foreach i in {1,...,10000} {

                    draw[thin,rounded corners=8pt] ({i*0.05},0) -- ({i*0.05},2) -- ({1+i*0.05},3.25) -- ({2+i*0.05},2) -- ({2+i*0.05},0) -- ({i*0.05},2) -- ({2+i*0.05},2) -- ({i*0.05},0) -- ({2+i*0.05},0);

                }

        end{tikzpicture}

end{document}

The following relationships for run times should be expected:

[First run with tikzexternalize] > [Run without tikzexternalize] >>> [Later runs with tikzexternalize].

Edit:
To get a feeling for the additional work the first tikzexternalize run has to do, compared to the plain run w/o tikzexternalized, you can study the log files in the figtemp folder.

As a very crude way of getting a benchmark for how long it should take, you can see how fast your computer is in creating an empty tikz image in a single pdflatex run:

documentclass{article}

%your preable as in the main document

begin{document}

section{First}

        begin{tikzpicture}

            coordinate (aa) at (0,0)

        end{tikzpicture}

end{document}

As this is (very) roughly equivalent to the extra work tikzexternalize has to do for each picture during the first run. So the first run should overall take:

t₀ + n t₁ seconds, where t₀ is the time it takes to compile the document without tikzexternalize and t₁ is the time the TikZ picture above takes to compile.