Utilizing keyed files adds overhead to system operation. The overhead comes from a number of elements:
More I/O is required to update an item.
More code is executed to update an item.
The additional I/O comes from having to insert and delete keys from the key file. In general, this only involves 1-4 extra disk drive read operations and usually only 1 extra write operation per key.
The additional code executed is the actual key maintenance code which is used to maintain and balance the key structure. The code has been optimized to minimize both system CPU and I/O usage and is so fast it is unnoticeable with most applications.
The overhead relating to key maintenance is directly related to the number of keys that are being maintained.
See Also
Using the BTREE File Structure System
Using Lookup and Browse Routines
Source Code to Lookup Routines
Stack Code Calculator Function