2019 iMac constantly gets "site is consuming too much memory" in Safari

Welcome to Apple Support Community.

I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as shown here: How to use the Add Text Feature When Post… - Apple Community. We will then examine the report for issues in your Mac.

Jack

Start by completely removing CleanMyMac from your computer using the developer's' instructions. Macs—like cats—clean themselves and have not needed cleaning software since about 2000. Any "cleaning" app WILL interfere with elegant, automated self- maintenance routines you paid Apple to program into the system. You can get odd symptoms like you describe when the interference is present. Let your Mac be a Mac.

Your computer was not starved for RAM at the time of the test, and the Fusion drive is nicely performing at expected speeds.

The WD software is unneeded and shows an install date older than the computer. Probably was moved over by Migrations Assistant,. It can go.

Capital One Shopping is crashing. So is Grammarly extension. As a Safari extension, that could explain your Safari issues. Try runnig with those

Spotlight indexing (processes starting with "md" ) are seriously overactive. They often work hard right after a restart. They can also work overtime if you have Spotlight set to index every last bit of info on your computer—that is the default setting. You can adjust the number of things Spotlight must index downward:

Change Spotlight preferences on Mac - Apple Support

Sometimes you may need to rebuild the Spotlight index:

Rebuild the Spotlight index on your Mac - Apple Support

but try that last.

Hello, GTR81.

Sorry that you're having trouble with your Mac.

Great job posting the EtreCheck report. It looks pretty clean to my eye, except for a single suspect.

I would suggest you disable/uninstall CleanMyMac and see if that has any positive effect on your issue. Remove it from your system per the developer's instructions.

Very often we hear from folks experiencing some performance issue that can finally be traced to one or multiple anti-virus/anti-malware apps installed and running on their machine.

Keep your software updated and macOS is capable of keeping itself secure. There is no need to run any anti-virus, cleaning, speed-up or optimizing apps. In fact these apps often cause more problems and headache than they prevent because they conflict with the macOS’ built in securities. If you have this type of app installed, you should uninstall it according to the developer's instructions, and use best practices when using your Mac.

EDIT: +1 to the above posted by Allan jones.

I failed to finish a sentence I see! 😳

Capital One Shopping is crashing. So is Grammarly extension. As a Safari extension, that could explain your Safari issues. Try runnig with those

That should say "Try running with those disabled or uninstalled."