Why is my iMac taking over six minutes to boot up?

In 21.5-inch iMacs made between 2012 and 2019, "slow boot" complaints here are almost always due to the slow and under-spec storage Apple chose for models that were not up-orderd to with optional, better storage.

If Safe Model boot does not help yours, we can give a data-driven evaluation in this setting where we can neither see nor touch your computer if you post post an EtreCheck report. We can quickly and within the limitations of these forums help you determine what issues are at play without our playing a protracted game of "20 Questions" with you that could go on for days. 

EtreCheck Pro is available here:

https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, possible software issues and interferences, and RAM usage. Etrecheck is the development of a long-serving and trusted ASC contributor. It is a reporting app, not a "fix-it” app, expressly for displaying information in these forums to help us help you remotely. It will not reveal any personal or secure information.

It is the single best tool we have for helping other users remotely.

Please see this excellent user tip on how to post long text reports like EtreCheck's into a forum response:

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

Please determine if the same problems occur in "Safe Mode": Start up your Mac in safe mode - Apple Support.

Safe Mode usually takes longer to complete its startup as it performs a cursory startup disk integrity verification and directory "repair".