Improve performance of Pipeline

[bertfrees]

This is just a reference to snaekobbi/issues#28. This way I'm adding the issue to Dedicon's to-do list.

• The system must be able to handle several productions a day;
• The maximum conversion time of one production must be 10 minutes;

+1 from:

• SBS: Improve performance of Pipeline sbsdev/pipeline-mod-sbs#52
• NLB: Improve performance of Pipeline nlbdev/pipeline-mod-nlb#55
• Nota: Improve performance of Pipeline pipeline-mod-nota#24

[dkager]

Currently seeing conversions that take ~45 minutes. Needs further investigation, e.g. is it the book size, the CSS stylesheet and/or the particular XML tags used?
A good start would be to try https://github.com/daisy/pipeline-mod-braille/tree/optimize

[bertfrees]

Note that the optimize branch is merged into develop. Forgot to delete it.