iMac running very slow

Welcome to Apple Support Communities.

You have adware. Use the free version of Malwarebytes for Mac to remove it.

Delete LaCie software. It is not needed.

Go through and delete the Internet Plug-Ins in /Library/Internet Plug-Ins/ and ~/Library/Internet Plug-Ins/.

Other than that, the reason your iMac is very slow is due to the 5400 rpm hard drive. If you use an external SSD and run macOS from that, it will make your Mac 10 to 30 times faster for storage. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

Cheers,

Jack

Taking a look at the report, here are my observations/suggestions:

• Hardware Information
  • The combination of an i3 CPU, 8 GB of RAM, & mechanical 5400 RPM HDDs is the core issue for overall "slow" performance of your iMac.
• Adware - Although Macs can't get viruses, they can get "infected" by adware ... and yours is. I suggest either reviewing what EtreCheck is offering to remove it, and/or, use a third-party anti-malware app, like Malwarebytes for Mac to find and remove any present on your Mac.
• Unsigned Files - Remove Silverlight. This is an outdated Microsoft service that is no longer used or required.
• User Launch Agents - Remove AdsCleaner. This is one of the sources of adware and is typical when bad actors offer adware prevention "cleaners" that actually install adware instead.
• Performance - You can see the limitations of using a 4500 RPM HDD here in the low Read/Write speed numbers. However, the ones shown would be typical and do not, represent by themselves, an issue with the drive.
• Virtual Memory Information - Note that the Swap Used value is not zero. That means, at the time of the report, there was not enough system RAM to hold all of the apps/processes in memory, but some had to be "swapped out" to the slow system HDD. These swaps are one reason for overall system "slowness."
• Clean up - I suggest that you follow the EtreCheck report recommendations to remove each of the identified items listed in this section.
• Diagnostics Information - The photoanalysisd High CPU Use item is basically telling you that this process (at, at least 2x) used a significant amount of CPU processing. This is not necessarily an issue. This process is used by the Photos app for facial recognition processing. If you enabled this feature in the app, and have a lot of "people" photos, this process will run to help identify/categorize them. Especially the first time you use the app or any time you add additional photos to the Photo Library.

Yes, outside of the limited CPU & memory, that is one of the "bottlenecks" that your system has. I also concur with Jack-19 on using an external SSD. Regardless, give both of our suggestions a try and the could go a long way to improving your Mac's overall performance. If you come across any issues or have any additional questions, just let us know.

Keep your current LaCie for Time Machine backups. You need a separate external SSD for macOS.

Jack

But basically I'm hearing I have a slow HDD.

Yes. The external SSD is the easiest way to fix that.

Also, for the external SSD, I would recommend a capacity of 1TB for you.

Jack

Hi - my Mac is running slow / beachballing

Using OS Monterey 12.1, 3.6 GHz w/ 8GB 2400 MHz.

EtreCheck is attached - if anyone can shed any light it would be appreciated