iMac working slow after macOS update

More than likely you have a third party app installed that is running on startup that is causing the problem. First remove the known categories of apps seen to slow your computer down::

• AntiVirus
• Optimizers
• Cleaners
• VPN

Then post the free EtreCheck report using the Additional Text option when posting. This will not only list the apps running on startup, but shows drive performance and recent crashes that may contribute to the slowness you are experiencing.

How to use the Add Text Feature When Post… - Apple Community

The differences is that the 21.5" iMac has a internal HDD or Fusion drive and the MacBook has an SSD.

EtreCheckPro is an excellent tool for checking hardware slowness and other problems.

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

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

If that is a 21.5" 2019 iMac, and you are booting off of either

• A 2.5" 5400 rpm mechanical hard drive, or
• A 1 TB Fusion Drive with a miserly 32 GB of SSD storage (vs. 128 GB in the original 1 TB Fusion Drives)

your startup drive isn't a very fast one even if it is working properly.

You could get those iMacs with 256 GB – 1 TB SSDs, but I'm guessing that most people who bought them only sprung for the hard drive or the Fusion Drive.

As you've been already asked to do please download and run Etrecheck.  The free version is sufficient.

Copy the report as shown in this animated screenshot

and use the Additional Text button to paste the report in your reply.

Then we can examine the report and see if we can determine the cause of the problem.

Please post an EtreCheck report of your system so we can look for obvious issues causing the problem.