I can't watch Youtube videos on my iMac

CleanMyMac is one of those things that can put a load on your system, and/or delete things it shouldn't. So you should at least launch

Applications -> Utilities -> Activity Monitor -> View (menu) -> All Processes -> CPU (tab)

And keep an eye on any processes that are consuming a lot of CPU. There are also Memory, and Networking tabs if you want to peek at them as well. Just make sure CleanMyMac is not consuming resources that should be going to transferring YouTube videos, and processing them via the graphics and audio drivers for display on your Mac.

I do not have anything against Malware bytes. It does not implement a firewall.

If you do not know what a VPN is, then it is unlikely you are using one. But to explain, a VPN is a "Virtual Private Network". The original use was to allow employees to securely connect to their company's internal network without bleeding all the company secrets across the internet. In the past few years there have been consumer VPN companies that claim to keep your internet browsing a secret from your ISP, from coffee shop owners, etc... as well as allow users in another country access country limited content by having the VPN exit node being the foreign country so the content distribution service thinks they are a resident of the country. For example watching BBC shows from another country. The only problem with this is, many of those consumer VPN services just vacuum up your web surf'ing data and sell it on the side.

What web browser are you using to view YouTube. Have you considered trying a different web browser?

• Safari
• Firefox
• Brave
• Chrome
• Microsoft Edge
• other...

Another trick to try "JUST AS AN EXPERIMENT", boot into Safe mode

http://support.apple.com/kb/ht1564

This will not load any 3rd party utilities (yes, CleanMyMac will not be running), but Safe mode is an easy way to see if 3rd party software is causing your issues. NOTE: Safe mode uses a much more conservative video driver, so you may see some lines flicker across your screen. As this is just an experiment, it is nothing to worry about.

Ok, to have a chance to resolve this, let me ask a few questions:

• Does this issue only happen with your iMac?
• Which exact model is your iMac and is it still running macOS Catalina as per your tagline?
• How is your Mac connected to your local network ... by Ethernet or by wireless? Regardless, could you please provide a bit more detail on each of your network components. Brand & models will help. How are they all configured to provide Internet access?
• Have you tried creating a new user account on your Mac ... and then, log into it to access YouTube? Did this issue persist?

After you power cycle,

• all the memory allocated to precesses is back in the free list
• All the web browses have a clean starting point
• All the networking stacks have a clean point
• Any 3rd party processes you have running in the background (CleanMyMac) have started with a clean slate

Basically everyone is starting from a clean starting point and does not have any cruft getting in the way.

Ok, thanks for the updates. By "Ethernet or Wi-Fi," I was trying to understand how your iMac is connected to your local network. That is, is it connected with a wire (Ethernet) connection to a router or Ethernet switch, or by wireless to your local Wi-Fi network?

If by wireless, is the iMac in the same room as the Archer router? ... or in a different room/floor from the router? This could make for a drastic difference in wireless bandwidth performance ... which in turn, would affect getting YouTube streams from the Internet. I'm trying to rule out if your Mac's network connection is the issue or if it is the Mac itself. One additional question: Does this issue happen no matter where you stream from the Internet to the Mac, or only for YouTube streaming?

Do you use a VPN? A VPN concentrates all your traffic at the VPN company's servers. Rebooting may just get you assigned to a different VPN server. NOTE: I am not a fan of VPN's, as it just concentrates all your traffic at the VPN where they can collect and sell it (especially free VPNs, because they have to make money somewhere).

Do you have an anti-virus package installed? (not recommended, but many people do it anyway). My company mandates a specific A/V product on the company Macs. The firewall has a tendency to slow down network connections. Sometimes users with gigabit connections are getting a few kilobits/second until they disable the A/V firewall. If you are running a A/V package, a reboot would reset the A/V's firewall, if it was responsible for your issues.