Page splits are expensive operations. In addition to the actual work of moving rows, allocating pages, and logging the operations, the cost is increased by:
Updating the clustered index itself
Updating the page pointers on adjacent pages to maintain page linkage
Updating all nonclustered index entries that point to the rows affected by the split
When you create a clustered index for a table that will grow over time, you may want to use fillfactor to leave room on data pages and index pages. This reduces the number of page splits for a time.
See “Choosing space management properties for indexes”.