Ultrafast PDFLaTeX with precompiling

"precompile" is probably a slightly confusing phrase to use as TeX is not a compiler but (mostly) a macro expansion language, but anyway...

In general you can dump most macro definitions and register assignments into a format. What you can't do is ship out pages. So in practice you can dump most LaTeX preambles.

Rather than having to edit the file so that it only works with the preloaded preamble format it is possible to leave the preamble as normal, but define the dumped format to skip the commands that were previously executed in the dump.

My truly ancient mylatex files on ctan do this or there is a newer version of that with additional features and better maintained: mylatexformat

The main thing you have to beware of (and which I suspect you are falling over) is if any of commands that you dump use \jobname (for example to open auxiliary files) then you have to ensure that the jobname when you dump the format is the same as the jobname when you produce the document. Also If the macros are assuming that files opened by commands in the preamble are still open when the document is processed then you will need to re-open them when you use your preloaded format loading the format file will re-set TeX's internal state with respect to its internal memory but it will not re-assign the file handles to the filesystem.

The first hit on searching for mylatex on this site shows an example discussing this in the context of tikz externalize.

TikZ's externalization and mylatex

For the benefit of people using a search engine arriving here looking for how to convert a never-changing but slow-loading preamble into a super-quick "precompiled" format.

The instructions here take you through a simple process, as follows. You need the mylatexformat package to make this work.

1. Split your preamble into "static" and "might change" stuff, with the static stuff first.
2. At the end of the static stuff add the command \endofdump
3. Run the following from the command line/terminal, replacing both instances of foo with the name of your document:

pdftex -ini -jobname="foo" "&pdflatex" mylatexformat.ltx foo.tex

4. Delete the static part of your preamble (might want to save it somewhere else for safe keeping!) (Note: it is suggested in the comments that this step isn't necessary. I haven't tested it but if someone does and confirms this, feel free to edit this answer and delete this step.)
5. Insert the code %&foo at the top of your .tex file, replacing foo with the name of your file.

Your compiles will now happen much faster.

Note: if you have any "might change" preamble, you need to keep the \endofdump command, otherwise compilation will start at begin{document} skipping all your preamble