Very Slow broadband completly specific to iMac

After each change to Mac OS you should be doing the following to verify:

1. Restart Modem
2. Restart Router
3. Restart Mac in Safe Mode (may take 10 minutes or slightly longer) and then restart normal.
4. Test

This would be considered more thorough troubleshooting than simply making a change and thinking it will work. It simply means you are starting from the same base point when making a change and giving the system the opportunity to "take" the change. Yes it is more tedious however it is more thorough.

Create a new admin user, login as that user and retest. There may be a problem with your current user settings.

A simple restart may fix the issue as might a restart of your router and/or modem. Strange as it may seem, restarting our router often fixes a device specific issue.

It is not likely a hardware issue. I recommend that you call AppleCare today and they can help log into the machine (use an Ethernet connection to get Internet) and do some diagnostics with you. You can find the phone number by clicking AppleCare Contact Info

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Make a New Location, Using network locations in Mac OS X ...

System Preferences>Network, top of window>Locations>Edit Locations, little plus icon, give it a name.

System Preferences>Network, click on the little gear at the bottom next to the + & - icons, (unlock lock first if locked), choose Set Service Order.

The interface that connects to the Internet should be dragged to the top of the list.

Sys Prefs>Network>highlight the Interface you're using>Advanced>TCP/IP>Configure IPv4: Using DHCP.

If using Wifi, instead of joining your Network from the list, click the WiFi icon at the top, and click join other network. Fill in everything as needed.

System Preferences>Network>choose interface>Advanced>Proxies Tab, make sure none are set, like for HTTP & HTTPS.

System Preferences>Network, unlock the lock if need be, highlight the Interface you use to connect to Internet, click on the advanced button, click on the DNS tab, click on the little plus icon, then add these numbers...

8.8.8.8

8.8.4.4

1.1.1.1

9.9.9.9

Apply.

Old Toad beat me to the punch, see his post on what a Safe Mode restart does, you can also read up on it in:

How to use safe mode on your Mac - Apple Support

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?