I have a ssd, and my computer boots up so fast, it just doesn't look cool, moreover, the time is so less (2-3 seconds) that the startup splash has no time to show, which sucks, how to delay startup to atleast 15 seconds, to make it look like some "beast" is starting up
Most users post asking how to speed up boot. Here you are... being sane.
My SSD also boots too fast for the Zorin Splash screen.
I used to be able to get a couple things done or organized before the computer boots. Now I have to jump at its beck and call.
You might try increasing the grub timeout
You want to slow down, interesting. When You mean the Grub Menu You can choose the Step from @Aravisian but when You are on the Grub Timeout You could set it to -1 because then it will only disappear when You make a Choice.
That is the base default, but a user can set the unit with s for seconds, m for minutes and even d for days. I do not recommend that last one.
The decimal would appropriately travel