Slow package building with rstan 2.26.23

[cdriveraus]

          > happens with the rstantools configure script disabled too, and no compilation occurring, fyi.

This is because we're using a less powerful javascript engine (quickjs instead of V8) which is more portable, but less performant.

There were significant optimisations in stanc3 after 2.26, so once we get StanHeaders 2.32 on CRAN then the parsing time should be ~3x faster

Originally posted by @andrjohns in stan-dev/rstan#1084 (comment)

If I've disabled the configure script of rstantools in my package, and the binaries are already built, is it true that parsing is still occurring? if so, can it be turned off somehow? I realise the aim is to get rstan 2.32 out quickly, but forgive me if I don't have full faith there ;) and even 3x faster will still be annoying for package dev...