Why does Tex/Latex not speed up in subsequent runs?

Question

I really wonder, why even recent systems of Tex/Latex do not use any caching to speed up later runs. Every time that I fix a single comma*, calling Latex costs me about the same amount of time, because it needs to load and convert every single picture file.

(* I know that even changing a tiny comma could affect the whole structure but of course, a well-written cache format could see the impact of that. Also, there might be situations where 100% correctness is not needed as long as it’s fast.)

Is there something in the language of Tex which makes this complicated or impossible to accomplish or is it just that in the original implementation of Tex, there was no need for this (because it would have been slow anyway on those large computers)?

But then on the other hand, why doesn’t this annoy other people so much that they’ve started a fork which has some sort of caching (or transparent conversion of Tex files to a format which is faster to parse)?

Is there anything I can do to speed up subsequent runs of Latex? Except from putting all the stuff into chapterXX.tex files and then commenting them out?

Answer 1

Yet another answer, not strictly related:

You can use the LaTeX macro \include{...} and with \includeonly{} you can rerun your document for a subset only. But this is not caching, nor does it give you the complete document.

Answer 2

Let's try to understand how TeX works. What happens when you write the following?

tex.exe myfile.tex

TeX reads your file byte by byte. First of all, TeX converts each char to pair <category, ascii-code>. Each character has category code and ascii code. Category code means that the character is an opening brace ({) or entrance into the mathematical mode ($), symbol-macro (~, for example) or letter (A-Z,a-z).

If TeX gets chars with category code 11 (letters) or 12 (other symbols: digits, comma, period) TeX starts a paragraph. You want to cache all paragraphs.

Suppose you changed something in your document. How can TeX check that all paragraphs after your changes is the same? May be you changed the category of some char. Me be you changed the meaning of some macro. Or you have removed } somewhere and thus changed the current font.

To be sure that the paragraph is the same you must be sure that all characters in the paragraph is the same, that all character categories is the same, the current font is the same, all math fonts is the same, and the value of some internal variables is the same (for example, \hsize, \vsize, \pretolerance, \tolerance, \hypenpenalty, exhyphenpenalty, \widowpenalty, \spaceskip, ..., ........)

You can be sure only that all paragraphed before your changes is the same. But in this case you must keep all states after each paragraph.

Your system SuperCachedTeX is very complicated. Isn't it?