Has macOS 15.3.1 made my iMac 2019 very slow?

One possible cause is that your system drive is too full. The free space ought to be at least 15-20% at all times.

There could also be some third party software that is installed and is causing problems.

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

With only 38 GB free out of a 1TB drive, you should consider it FULL.

Always strive to keep 15-20% free on your system drive.

Oh... less than 40GB free on a 1TB drive, boy that is FULL!

Try to clear some space as soon as possible.

PFJ30 wrote:

Very slow response times for all sorts of things that are usually instantaneous eg. click on About this Mac took longer to open than writing half this sentence.

The current stable release of Sequoia including bug fixes, security updates is macOS 15.3.2 - I would start there.

Keep your Mac up to date - Apple Support

Keep your Mac up to date - Apple Support

An aside to the main issue

Which @Luis S 👍 has very rightly identified as the Third Party Replacement EVO SSD is basically totally full

The Etrecheck Application used for this report is way outdated EtreCheck version: 5.7.2 (5247)

This should not affect the reliability of the current report Nor will it change the Real Issue, the Drive is Full

Though moving forwards

The Current Etrecheck application from the Developer is

PFJ30 wrote:

Thanks. I presume 15.3.2 is Beta, no? I usually avoid them...

15.3.2 is not beta anymore, it was released very recently (yesterday or so).

But I don't expect the update to solve your problem, because I don't think it is a bug, but something specific to your system.

PFJ30 wrote:

How do I give etrecheck Full Drive Access?

There are two steps for this: first in System Settings->Privacy & Security, click Full Disk Access then click + and select Etrecheck to grant it permission. Second, when running Etrecheck there is a setting you can turn on or off for “Full Drive Access”. I am not at my Mac now so I can’t check the details but it is not hard to find.