How do I improve a slow iMac 2019 that has low CPU and memory usage?

Run DriveDx (free trial period) to check the health of the internal hard drive. Post the complete DriveDx text report here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar. A worn out or failing hard drive is a real possibility for a six year old system. You may as well post the report for the internal SSD as well.

Your Anker USB hub is also is only showing as a USB2 device so if you use it to connect any external storage, then that will be a huge performance hit for those storage devices since USB2 has a maximum transfer rate of about 45MB/s.

With only 8GB of memory, it will really stress your system while you are using a VM. macOS itself really needs at least 8GB of memory just for itself. If you use the VM a lot, then adding another 8GB or 16GB of memory would be a good idea. Since Macs are very picky about the memory they use, it is best to only get the memory from OWC or one of their authorized resellers/partners...unfortunately it seems Crucial may no longer be selling Mac compatible memory. If you have the performance issues even without the VM running, then memory probably won't help with those performance issues since they would be caused by a hardware failure or some other issue with third party software. Wait to purchase any memory until we can resolve the system performance issues...no use spending money if it is not necessary.

Also, I agree with @PRP_53 regarding CMM even though it appears it is only a login item here. I didn't really see any other issues with third party software stand out to me, but I'm not a macOS software expert.

garethtrussell wrote:

Thanks. I’m not too worried about the HDD — it was in place at the beginning and wasn’t noticeably slowing everything.

Will uninstall CleanMyMac. But again, that had been used to try and improve a slowing system.

What would be the best way to deal with system modifications?

To be brutal about this situation

Once CleanMyMac has been installed / used on the computer coupled with the rather Uniques Software configuration appearing in the report

Added is the Fusion Drive which is and going to be a big impediment to performance

RAM - 8 GB is the bare minimum and can be upgraded

I might suggest several things

1 - Start Over from Scratch

2 - Get a worthy External Enclosure with a Large SSD Drive www.owc.com is a good starting point and yes they do ship Internationally

3 - Install the Operating System to that drive and use it as the Startup Drive

4 - RAM - 8 GB RAM - Upgradeable by an Apple Authorized Service Provider to at least 16 GB RAM

5 - Save your Money and Time. Consider the purchase of an ALL SSD configured machine with at least 16 GB Unified RAM or more

APPLE HDD HTS541010A9E632 1.00 TB (Mechanical - 5400 RPM) 

APPLE SSD SM0032L 28.00 GB - Small

 macOS Sequoia 15.3.1 (24D70) 

/CleanMyMac_5_

A - Fusion Drive and 8 GB RAM running macOS 15 Sequoia 

A - CleanMyMac aka BrickMyMac 

A - System modifications - There are a large number of system modifications running in the background.

A - Do you what the extend the usefulness of this computer for a couple more years ?

Follow on

Under the current and Known configurations

This machine has reached is' Zenith for Upgrades to newer version of macOS

We do Not Know for Sure what will be the Cut-Off Point for Qualifying computers for the Successor to macOS Sequoia

Just saying