Why does my Mac boot up so slow?

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

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

Then we can examine the report and see if we can determine what might be causing the problem.

I don't see any of the usual suspects in the report. However, you have the slowest iMac offered buy Apple at the time of purchase: 8 GB RAM and a 5400 rpm hard drive.

With that in mind I would remove all apps from the System/Users & Groups/Login Items preference pane and manually launch them only when needed and close when done.

Your read/write speed tells the tale:

Performance:

Write speed: 73 MB/s

Read speed: 84 MB/s

You can increase that and the speed of opening/closing apps and saving documents by ups to 35 times by getting an external Thunderbolt SSD, cloning your boot drive to it and booting and running from it. This MacSales.com drive is an excellent candidate: WC Envoy Pro EX - Thunderbolt 3.

Or for less money and up to about 12 times the speed this is a good candidate: Envoy Pro EX with USB-C

How slow?

What apps do you have starting at log-in?