What is the relationship between purchase workflow process time duration and churn rate?

I am fully aware that the shorter the path to purchase the fewer people you will drop in that process. However, is there some study made on that?

I am looking for a chart for online store checkout process dropouts ranging from amazons single click purchase to some half an hour call a landline talk with a manager type of store.

I have a client that that may need to add some security checks on their mobile app checkout process and I would like to provide them with data-driven estimation of how much will that cost them.