Resetting iMac

It is suggested before going the Factory Reset Route

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

Is the computer configured with a Fusion Drive ( Combo SSD Drive with a Rotational Drive ) ?

This type of configuration would be presented to the User as a Single Drive through the magic of Apple to Fuse the two as one Drive

Sometimes the " is running real slow. " is more related to the Software installed and used on the machine

To avoid the question / answer and question answer scenario

Suggest the User start with Points 1 to 4 and then Test

Still having issues, move onto Steps 5 and 6 and Test Again

Still no resolve - move onto Step 7 and follow instructions therein 

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Safe Mode will also eliminate Third Party Software, extensions and drivers from loading. It will only load the Minimum amount of Core Apple Processes to allow the the computer to function at a reduced Level of Performance 

3 - Does the issue present in this mode ?

4 - Sometimes a Safe Boot followed by a Normal Boot will just put things right.

5 - If not - there could be something in the main User Account playing up. To further isolate this - Set up users, guests, and groups on Mac. Then log out of the Main User account and log into the dummy account and test again if the issue persists.

6 - If the issue is present in the dummy account - then, this appears to be a System Wide issue on the computer.

7 - Download the Application Etrecheck directly from the Developer.

The Application is Not a " Silver Bullet "  and is  only a tool to examine the Hardware / Software used on this computer 

This is a Diagnostic Tool that makes no changes to the computer Hardware / Software used on this computer 

The application is free or paid from added features. 

The Report will Not Reveal Any Personal Information. 

Post back the Full Report - copy and paste - >>>> using the Additional Text Icon ( 3rd Icon to last ) <<<<

Q - " Hi, I have an iMac 2020, i5 with 1TB drive (hardly used).....  I want to do a factory reset as it is running real slow... I have exhausted a million checks and tests and Apple phone support couldn't help either.  Whilst I only had a few apps on it - mainly office suite, i do have a browser with loads of bookmarks and passwords for websites that i don't want to lose..... "

Q - " should passwords and bookmarks automatically reinstate from iCloud when i add AppleID on set up?  "

A - To answer the above questions.

Provided you have iCloud enabled and ALL the elements of iCloud enabled  the bookmarks and passwords from iCloud should come back to the computer after doing a Reset of the computer

Sign in to iCloud on all your devices

Apps and features that use iCloud

How iCloud keeps information up to date across all your devices

If I may.

Just purchased a new M2 Computer and set it up on Saturday Past.

I use ALL Elements of iCloud that Apple offers and all the Safari Bookmarks came back as did all my Keychain Passwords 

Q - " I am relatively new to Apple, but should i use the 'Time machine' to do a back up? and will it remember the above?  Does a time machine restore put my apps back on? "

A - Yes for sure do a Time Machine Backup  before doing a Reset of the computer.

If possible, do a second Time Machine Backup to another dedicated external Drive .

Back up your Mac with Time Machine

Backup disks for Time Machine

Having more than one TM Backup is good incase the first drive fails

Q - " I would appreciate some comments as to best way to reset it. I don't want the reset where it keeps personal files intact as this is presumably not fully factory resetting it... "

A - This article from Apple   has all the needed information. Though it says Sell, give away etc the method to Reset the computer will apply to your needs 

>> What to do before you sell, give away, trade in, or recycle your Mac

You may find some better performance after the Reset is performed

Though be aware Fusion Drives like this one in the report are not going to perform as well as a totally SSD Internal Configuration

The small SSD 32 GB and a Slow 1 TB 5400 RPM Rotational / Mechanical Drive will perform but struggle under the demands or Sonoma macOS 14 

 iMac (Retina 4K,21.5-inch, 2019)

Drives:

disk0 - APPLE SSD

SM0032L 28.00 GB (Solid State - TRIM: Yes) 

Fusion drive

HTS541010A9E632 1.00 TB (Mechanical - 5400 RPM)

Am working on my iPhone right now

Will have a good look at the report Friday early AM my time EDT

Yes 2019 is just fine and Yes it is a Fusion Drive as per the report

Should another Contributor pick up your question and review the report before me, that is good too 👍