By default DesignBuilder recalculates zoning and surface adjacencies after partition is drawn and in large complex models this can slow things right down. This process can be speeded up by postponing the rezoning to a single operation after block partitioning has been completed. To do this, open the Model options dialog and switch off the Automatic block zoning option under the Miscellaneous Edit header on the Advanced tab. When partitioning has been completed, the block can be manually re-zoned by selecting the Rezone block command from the Tools menu. Partitions drawn with the Automatic block zoning option switched off are called Outline partitions and are shown in blue.

For more on this see:

http://www.designbuilder.co.uk/helpv2/Content/_Drawing_partitions.htm