For example, if it takes more than 24 hours to backup or restore a database, obviously this presents some problems. What can sometimes happen to the DBA is that a database may be of reasonable size now, given their current server hardware, and backups and restores may take a reasonable amount of time. But at time goes by, the database grows, and at some point the amount of time to backup and restore the database becomes too long given the backup and restore window available. The moral to this story, keep a close eye on backup and restore times, and factor this in when projecting future hardware needs for your SQL Server databases. For example, full backups take the longest to perform, but are the fastest to restore.
Differential backups are overall faster than full backups, but take longer to restore.
This is because the restoration of transaction logs have to play back each transaction when restored. If a transaction took 5 minutes to run, it will also take about 5 minutes to run again when the transaction log is restored. Backing up a database directly to tape over a network is generally slower. For maximum backup speed, the local array you back up to should be RAID 1 or RAID 10 due to the high percentage of writes going on. All devices should be on the same high speed switch.