Build time

[JulienPalard]

I ran a quick test on docs.p.o, now that 3.10 doc can be compiled in parallel, for a single build (one language, one version) :

HTML only it takes 2mn in parallel vs 5mn single threaded.

Full (with pdf, epub, texinfo ,,,) it takes 26mn in parllel and 38mn single threaded, it boils down to:

• html 2%
• text 2%
• A4 latex 27%
• letter latex 29%
• epub 13%
• texinfo 25%

I think it would be interesting to dive into this to see if it can be enhanced.

[hugovk]

This can be closed in favour of the newer #169.