How do you decide wether or not to charge for a new functionality(saas)? it improves productivity, but it also helps lower time to value for certain kinds of users. How do you weigh up the relative value of shorter time to value vs MRR?

Whenever we launch a new product or feature, I make sure we're aligned on the core business metric we want to drive. Is this a churn reducer? A top of funnel acquisition play? An expansion or ARPU play? This should help to guide how you frame the opportunity. 

If you're torn between a few potential paths forward, I always suggest a quick back of envelope calculation to help size the opportunities.

In this case - if you were to offer it to all customers for free, how much could you improve the activation rate (or time to value rate)? By doing so, how many more customers would you convert or retain? Put a dollar amount to that. Then take the flip side. If you charged for this as an add on, or included is in a higher package, how many customers would purchase? Now you have the increased MRR from that group, as well as the improvement in churn/activation for that (probably) smaller cohort. 

With both calculations, you can weigh the pros and cons.