In comparison to LaTeX with LuaHBTeX, ConTeXt with LuaMetaTeX is
lightning fast.

pdfTeX is 8bit, and Knuth’s plain TeX is very low level – for a fairer
comparison, try the TeXbook on LuaMetaTeX (I don’t know if “plain” works
though).
 
I understand that why it is slow, but the problem is that the difference is too high.
My book has lots of math, so perhaps that is the reason.

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Respect,
Shiv Shankar Dayal