My mac is starting slow. I know login items, CleanMyMac. What else? My MAC has a slow start-up time (at times, 5s or more before initial black screen with Apple logo).
I am aware of login items. I have used CleanMyMac. Is there anything else I should check for?

Thank you :)
CleanMyMac should be used with caution, or not at all. It is not recommended as a general trouble-shooting step. I would recommend EtreCheck , which provides you with a useful report about problems, incompatible software, and other concerns. You could edit your question to include any items that it flags. (Don't just copy and paste the whole thing, as it's quite long.)

Firstly: What's your storage like? Mechanical hard drive? How old? How full? What model of Mac, e.g. "Late 2012 13" MacBook Pro". What MacOS version?

Are you seeing slowness at any other stage? Problems that early are unlikely to be caused by user software.