iMac is slow after the Tahoe installation.

are you using any apps that claim to "clean up" or "speed up" or otherwise "optimize" your Mac? and / or any antivirus apps? the only thing those apps are good for is causing major issues within macOS while providing ZERO protection or benefit. you would do well to uninstall them.

if you have no such apps, i'm thinking you should download and run the free version of EtreCheck so we can see if you have some software installed that is causing your issue. make sure you give "full disk access" to etrecheck. Learn how to use it by reading Using EtreCheck. if you need help interpreting the report, you can see how to post the report here by reading How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report. and it automatically obscures sensitive things (like serial numbers) so you don't have to worry about sharing the report here.

If you mean Erase all Content and Setting ?

If doing that procedure, it would have only removed everything Except the Operating System, Tahoe 26.1

It would have remained untouched

If you had since restored the computer from a recent TM Backup when issues were presenting

The Restore from the backup would have Re-Introduced the existing Issue right back into the computer

Thereby, Negating all the work you had done previously

You best course of action is to follow the good advise from my colleague @ jefferythefrog and use the Etreach application and post the Full Report

From Within the Etrecheck application

You " Copy Report "

This will place a Copy of the Full Report onto the Clipboard

ie Finder >> Edit >> Show Clipboard. Check of it is there

Manoeuvre to Additional Text

⌘ + V as in Victory will paste the report into Additional Text

Give it a Name like Etrecheck Report

Once done >> Post

As a " Catch Me All " and " Blanket " statement

Barring the Etrecheck Report being successfully posted

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 

like Lulu and Little Snitch 

2 - Commercial VPNs 

They may not be what one believes they are doing for the computer and what they actually are doing behind the scenes and unbeknownst to the user ( you ) 

They may also reduce your Internet Speeds by upwards of 30% 

https://gist.github.com/joepie91/5a9909939e6ce7d09e29

VPN: What you need to know - Apple Community

3 - Third Party Security Software 

There are no known Windows-like Viruses in the wild that self replicate and affect macOS, because of the underling UNIX  Foundation and Permission Limitation. 

The Operating System resides in a Sealed and Read Only Volume that cannot be opened by the User nor by Third Party Applications.

The Only thing this Antivirus software is protecting is the Bank Account of the Developers and for zero return to the User aside from the problems this software creates.

Security. Built right in

Mac app security enhancements

The Built in Security  is all that is required to protect the computer.

Protecting against malware in macOS

4 - Third Party Disk Cleaners / Optimizer 

Any Third Party Applications that will interfere with the normal operation of the OS,  is an invitation for disaster. 

Certain Applications maybe available on the Apple Apps Store - this only means the Developer is prepared to pay Apple a percentage on each sale. 

What the Application may do to the computer is up to the User to check this out before purchase

To put the Third Party Disk Cleaner / Optimizer in context and the damages it may have or has already done.

This type of applications can or will Muck Up your User Account ( Home Folder ) of this machine.

It does not touch the Operating System itself unless you consider your User Account ( Home Folder ) as part of the Operating System.

The Operating System may be  hosed