Why is my 2019 iMac running so slowly?

Tried using external hard drive to boot..

To see improvement, the external must be a minimum SATA6 solid-state drive (SSD) and its enclosure must be at minimum USB-3. A conventional hard drive in an external enclosure can be no faster than your current internal drive.

An EtreCheck report will include drive performance data.

To prevent us from playing 20 questions,

Download and run the free version of EtreCheckPro, from > https://etrecheck.com/en/index.html

Then post the Report, as per > How to use the Add Text Feature When Post… - Apple Community

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