Why VM snapshots are affecting performance?

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http://serverfault.com – I read in one of the VMware KB article says that snapshots will directly proportional to VM performance. But my team keep asking me how snapshots can affect performance. I would like to give them solid reason behind the statement that snapshots are performance killers. Can any one explain a little bit theory behind why actually snapshots are affecting the performance? Is it just because Disk I/O rate of hard disk would be slow? (HowTos)