activity monitor - what to delete?

You should only close the ones in the Dock, ones which you're familiar with, or ones you're instructed to close. If a specific background task is slowing down the computer, it's better to post here with the names of the ones taking up large amounts of resources or using other methods of speeding up the computer.

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Activity Monitor should be used to identify problem areas that should then be investigated. Excessive CPU, excessive Memory, excessive Power usage, excessive Disk I/O, excessive Network I/O, etc...

Posting the output from EtreCheck will often times identify 3rd party additions that frequently are responsible for system slowness

<https://discussions.apple.com/docs/DOC-6174> or <http://etrecheck.com>