The amount of memory used for buffers during backup directly affects backup speed, primarily for tape backups. The BLOCK FACTOR parameter of the BACKUP command controls the amount of memory used. If your backups are slow, you may want to increase the value of BLOCK FACTOR for faster backups.
The effect of BLOCK FACTOR depends on your operating system, and on the block size specified when the database was created. The default IQ page size of 128KB for newly created databases results in a default block size of 8192 bytes.
Set BLOCK FACTOR to at least 25 (the UNIX default). With this combination, BACKUP is able to buffer data ideally for most UNIX tape drives, with enough data in memory that drives are kept busy constantly throughout the backup.
On Windows, the default BLOCK FACTOR is computed based on the database block size. This value usually achieves maximum throughput on Windows. Because of the way Windows handles tape devices, you may not be able to achieve faster backups by increasing the BLOCK FACTOR.