My iMac is so slow but I have plenty of hard drive space.

The biggest problem I see is you bought a base model iMac that has 8GB of not-upgradeable RAM and an extremely slow 5400 RPM HD. These machines were designed for doing e-mail and surfing the web and not much else. However I see you are attempting to use it as a gaming machine. About the ONLY thing you can do with this computer is get an external SSD and then install Mac OS on it , then migrate your data and apps to the SSD. Then use the internal drive for additional storage. That's about all you can do with those computers.

On first look, your Mac has a terribly slow 5400 rpm hard drive which will do everything at least 30% slower than a 7200 rpm drive. additionally, you cannot upgrade the RAM. And, it mentions that there are icloud transfers waiting, you need to remember that uploads are a lot slower than downloads, so your cloud uploads may be choking your resources.

I see you have Dropbox installed and running. A growing number of users have been reporting it as being a resource hog as well as a security risk. A number are recommending Sync as it's more secure.

You might disable Dropbox and check the performance for a while to see if it has any effect.

Your welcome, if you can, I'd suggest getting a 27" iMac, at least these have up-gradeable RAM and if/when you get one, get it with a SSD.

We are waiting, if you didn't know please post the results per the diagram below:

The devices that are getting transfers still have iCloud turned on. You have to do that for each device. As this is a different subject, you may want to create a new thread.