[O] BUG? WAS: Re: Painfully Slow Export

[Berry, Charles]

tl;dr: `:exports results :noweb no-export :eval never-export' needlessly 
expands <<noweb>> code causing massive slowdowns on export. 

> On Jun 13, 2018, at 9:42 PM, Ken Mankoff <address@hidden> wrote:
> 
> One more thought - even if your system is significantly faster for some 
> unknown reason - why is export taking ~15 seconds for 200 blocks *when you 
> aren't exporting any of the code*. 
>  Why is <<noweb>> expansion occurring if ":eval never-export" is set? There 
> seems to be something inefficient going on here.
> 
>   

Using your ECM, C-c C-v C-i on your `baz' block. Note `:exports code'

,----
| Name: baz
| Lang: sh
| Properties:
|       :header-args    :eval never-export
|       :header-args:sh         nil
| Header Arguments:
|       :cache          no
|       :eval           never-export
|       :exports        code
|       :hlines         no
|       :noweb          yes
|       :results        verbatim replace
|       :session        none
|       :tangle         no
`----

Setting `:exports none' speeds things up a bit more. 

Looking back at your earlier posts, I see that you had `:exports results' for 
your python block but not in your ECM. i.e. you need these header args:

#+PROPERTY: header-args :eval never-export :exports results :noweb no-export

to show how bad things can be in your ECM.

The `:exports results'  will *really* slow things down. And this *may* be 
considered a bug. Babel isn't quite smart enough to sort out that you want 
results, but do not need to eval the src block nor check the hash in 
`org-babel-sha1-hash'.

Making matters a bit more complicated, I'd have thought that this

        :noweb (if org-export-current-backend "no" "yes")

would be enough to fix things, but it fails on recursive noweb expansions if 
you try to execute a src block interactively. And maybe that is a bug, too. 

HTH,

Chuck