my Mac is very slow

Safe Boot from the HD, (holding Shift key down at bootup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.

EtreCheck is a simple little app to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac.

http://www.etresoft.com/etrecheck

Pastebin is a good place to paste the whole report...

https://pastebin.com/

hold the Alt/Option key while clicking the WiFi icon on the main menu bar,

make a screenshot of all the displayed information,

post that screen shot here.

We will look at it to see the information/quality of your internet connection.

😁 danke, gute Hintergrund.

but this was not what i expected: A screenshot of only a part of the screen you can make with Shift+CMD+4 instead of Shift+CMD+3.

But the picture is good enough to read the Wifi list: You have a good connection, so it is not the WiFi that is causing this issue.

Now try the so-called Safe Mode that BDAqua proposes:

shutdown the mac, then start it again while holding the Shift key, it will be a very very slow start. The safe mode is a startup with only the most necessary parts of the OS, and it clears almost all caches. When fully started, log in as usual. Then do something and verify the speed of operation, make a note of it (even slower, or faster).

Then restart normal.

Post your answer. Tell us also the year of your iMac.

(I am not available for some time, thus our BDAqua (highly respected expert) will continue to follow this).