Oracle Online Backups. Impact of many tablespaces being offline

In looking a most implimentations of the Oracle Backups with the database up, I see that each tablespace is placed into backup mode in turn, backed up, then placed online again.

What is the impact in doing this to many tablespaces at once? If the system is in a light load or mostly query load would it make any difference? I am looking at ways to more efficiently run backups.

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