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.

-- 
Respect,
Shiv Shankar Dayal