2020 iMac slow and buggy after updates 26.5 and 26.5.1

Predicated on your byline " iMac 27″, macOS 26.5 "

This machine , which by all right, should be just flying along very nicely with Tahoe 26.5.1 baring a hardware issue

iMac (Retina 5K, 27-inch, 2020) - Technical Specifications

I concur with my colleagues ( @den.thed ) suggestion in order to avoid a game of 20 Questions

There are 4 Categories of Third Party Software / Services that are not needed 

Get rid of them via the Developers Instruction 

In no special order 

1 - Third Party 2 Way Firewalls 

2 -Commercial VPNs 

3 - Third Party Security Software 

4 - Third Party Disk Cleaners / Optimizer 

EtreCheckPro is an excellent tool for identifying slowness and other software issues.

Download and run the free version of EtreCheckPro, from > https://etrecheck.com/en/index.html

Then post back here with your Report, as per > How to use the Add Text Feature When Post… - Apple Community

When I upgraded my 2020 27-inch 3.8 Ghz Core I7 iMac (40 Gb Ram, 1 TB ssd) from Sequoia 15.7.7 to Tahoe 26.5.1, there was a noticeable and general slowing down of the iMac. Before the upgrade, I booted the Mac into Safe Boot mode and then into Recovery mode where I ran Disk Utility First Aid on the internal SSD boot drive and then on the Time Machine drive.

Tahoe is a larger operating system than Sequoia (thank you Apple) with more demand on the Mac hardware. I have not however, observed any obvious "bugs."

I do not install any anti-virus, games, memory management, or so-called "Mac Cleaner" applications on my Macs, nor do I use any third-party VPN software. My external Time Machine drive is a 2 TB Crucial X8 SSD, not a slower rotational drive.