Storyline - Capability to have each slide timed?

Do you mean you want to know how long the users spend on each slide? 

If the course navigation is Restricted, the NEXT button won't be enabled until the Timeline ends (unless triggers override that). Thus, the minimum time they spend on each slide would be the length of the Timeline. That defaults to 5 seconds. It automatically adjusts if you insert a longer audio or video clip. And you can adjust the length manually. 

• The program can calculate the duration based on the length of all the Timelines. That's in the Project Info when you publish: 

• Of course, someone might spend more than the minimum required time on a given slide. 
• FYI: Here's the User Guide info about restricting navigation: Customize the Menu in Storyline 360 - E-Learning Heroes (articulate.com) 

If navigation isn't Restricted, you'd need JavaScript to determine how long the user actually spends on each slide. (Sorry, I can't help with that.)