imac slow booting up!

The first place to look is in System Preferences - Users & Groups - Select your account - Startup items. If you have items in there that launch on start up that will slow down the system.

If you have installed any third party antivirus, cleaning or any other utilities please locate the developers uninstall instructions and uninstall. Then restart in Safe Mode ( Safe Mode Reset ) and then restart normally.

If those steps do not help your situation please let us know and we will have you do an EtreCheck report of your system to see if there is something that you have installed that is causing the problem or if there is a possible hardware issue such as a failing drive.

Welcome to the Apple Support Communities.

I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as shown here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack