O Excel não usa mais de um núcleo por padrão.
Se você tiver uma CPU de 6 núcleos com hyper-threading, consulte: Sobre o "Ativar processamento de vários segmentos" do Excel opção .
Do link acima:
Excel's "multi-threaded processing" option was first introduced in Microsoft Office Excel 2007. The setting is located in Excel Options | Advanced | General. It is enabled by default but must have a capable processor to be utilized. The "Enable multi-threaded processing" setting controls multi-threaded operations other than calculation. These include multi-threaded sort, row height calculations, and finding the best fit for column widths. Additionally: this setting is not available in the Object Model for use in VBA.