Calc's help pages should contain a high-level description of Calc's multi-threading to optimise performance.
Moving to NEW
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/2159c0c2f6fcef3d74be2fb609818e035b95ecb2 tdf#132784 Add reference to Calc Multithreading