In the Revit.ini file, settings in the [PerformanceOptimizations] section improve the performance of the software.
By default, the software uses multi-processing for wall join cleanup on all 64-bit computers, and on 32-bit computers (for Revit LT) that have 8 or more cores.
To disable multi-processing for wall join cleanup, set ParallelWallJoins=OFF.
[PerformanceOptimizations] ParallelWallJoins=off