Speed up rendering : enable parallel rendering

[kikislater]

Parallel rendering is disable by default. Rendering geopoppy 2 project is realy slow without this option.

QGIS_SERVER_PARALLEL_RENDERING

Activates parallel rendering for WMS GetMap requests. It’s disabled (false) by default. Available values are:

0 or false (case insensitive)
1 or true (case insensitive)

https://docs.qgis.org/3.4/en/docs/user_manual/working_with_ogc/server/config.html#qgis-server-parallel-rendering