imac internet very slow

Okay. A couple of questions. By chance do you have any third party antivirus, cleaning or ANY other third party maintenance type apps installed on the computer? If the answer to both questions is "no" please carefully read and execute the following:

Please 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

Understand all other devices are working okay. Please try the following as your first step. (BTW, when you tell us things like "I have tried every single fix I can find...." that tells us nothing. Please try to be specific when posting)

• Turn off modem for 2-3 minutes and restart.
• Turn off router for 2-3 minutes and restart.
• Reset the NVRAM on the iMac ( Reset NVRAM or PRAM on your Mac - Apple Support ) and then re-test.

If for some reason that does not fix the problem, please let us know and we can suggest other items to try.

I don't see any of the normal suspects so lets take a look at your DNS settings. Please look over DNS Settings Update and change your current DNS settings. If you are still using the settings provided by your ISP that could be the issue.