iMac running slow with plenty of space

There can be number of reasons why your Mac runs slow, but space (storage) doesn't cause slowness. Lack of memory (RAM) does. How much do you have installed? Do you have an older mechanical drive? Is your computer old and in the process of dying? Have you installed any antivirus apps or Mac cleaner apps? Do you ever restart your computer? Have you emptied the trash regularly? Do you use apps which require large amounts of RAM? Do you close apps you are not using? Any or all these things can be factors. So, unfortunately, no one here can answer your question since you have told us nothing about your computer, except that you have plenty of space. We would recommend that you download, install and run the free app EtreCheck. Be sure to enable full disk access. Save the report and post it down below under "Additional text". Then someone here can review it and offer some useful suggestions.

+1 for etrecheck...

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

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

You can save us asking dozens of questions by submitting a report from this App for us to look at…

EtreCheck is a FREE simple little diagnostic tool 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. It will not display any personal info.

https://www.etrecheck.com/

Pastebin is a good place to paste the whole report if you capture the URL while there…

https://pastebin.com/

Whew, they've changed pastebin & made it harder, but after pasting in, click Create new paste button, then Embed button, then copy the URL...

<script src="https://pastebin.com/embed_js/KuvnghqA"></script>

The important part is...

https://pastebin.com/embed_js/KuvnghqA

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...