Uwe Ricken continues a series on heaps in SQL Server:
The PFS page “can” become a bottleneck for a heap if many data records are entered in the heap in the shortest possible time. How often the PFS page has to be updated depends mostly on the data record’s size to be saved.
This procedure does not apply to clustered indexes since data records in an index must ALWAYS be “sorted” into the data volume according to the defined index value. Therefore, the search for a “free” space is not carried out via the PFS page but via the value of the key attribute!
Read on for more detail.