How can I speed up my iMac's reboot process?

Please read the following user tip and restate your question/problem in detail: 

     Writing an Effective Apple Support Communities Question - Apple Community

What model and year is your Mac?

What system are you running?

Please download and run Etrecheck.  The free version is sufficient. Be sure to give it Full Disk Access.

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.

if its an old iMac that's normal if its a new one I would recommend going to an Apple Store and let it check