Multi thread support is an experimental feature available on nightly build (and needing to be enabled). The bug tracking it is interesting to read, it has been

Multi thread support is an experimental feature available on nightly build (and needing to be enabled). The bug tracking it is interesting to read, it has been a long journey... https://github.com/openscad/openscad/issues/391 https://github.com/openscad/openscad/issues/391