Desktop Mac really slow start up

If your Mac is otherwise running normally, You can do the following. Open the System Preferences app. Click on the General link on the left, then Login items on the right. These can be selected apps which open at startup. Turning off all but essential ones will speed up the startup process.

There really isn't any way of us accurately telling you what may be wrong without a report of your system. Please read the following and execute the instructions:

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community It is helpful to us if you download the report to your Desktop.

Great reply so far. may also want checkdrives a bi deper...

You can use Drive DX to possibly get a better view of Drive health…

https://binaryfruit.com/drivedx

Thanks to Jack-19…

It may be wise to run SMART Utility to check the drives' health.