iMac OS 10.13.6 will not boot in Safe Boot

A Safe Mode boot can take up to 10 minutes of continuously holding down the Shift key. Have you done the reset correctly?

Well, if the Mac is slow, we can start identifying issues by creating an EtreCheck report. However, please restart into normal mode, not Safe Mode, for this.

EtreCheck is a useful app to diagnose 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 seen here - https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Cheers,

Jack

+1 Post an EtreCheck report of the computer.