why is my mac so slow?

Before running the Etrecheck report set Etercheck's preferences to Full Disk Access

and also in the System/Security & Privacy/Privacy preference pane.

My guess is you bought a base model 21.5" iMac with 8GB of RAM and a 1TB 5400 RPM HD. These machines are designed for doing e-mail and surfing the net and not much else. The RAM is not upgradeable nor is the HD and these machines are simply very slow performers.

If I am correct, the only thing you can do is to get an external SSD (USB 3.1 or if your iMac has USB C get a USB C external SSD) and clone the internal HD to the new SSD and then use the SSD as your primary drive and use the 5400 RPM drive as additional storage.

However I'd also recommend you do the following:

Please navigate to the Mac App Store or navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211