Clustered Index during data load in MS SQL Server
It's important to note that a clustered index isn't really just an index, it is also the physical structure/ordering of the pages that comprise the table, not just the B-Tree of the primary key.
This is one of the areas why Sybase/SQL Server sort of dropped the ball in naming, while other vendors (IBM/Oracle) did a better job referring to them as &quot;Index-organized tables&quot; which IMO illustrates the concept more clearly.
So if you drop the clustered index, the table turns into a heap.  When you re-enable it, the table has to be physically rebuilt, which requires sorting data and moving pages - both expensive operations.