The setting can be found in the General tab of the Journey settings (from the Journey, click Options & Edit > Edit User Journey):
There are three settings to choose from:
The User Journey will wait for the whole page to load (including all dependent resources such as stylesheets, scripts, iframes, and images). The browser will wait for the load event to return. This is the default behaviour and recommended for all websites that are not experiencing any timeout issues or for anyone that wants to understand when resources are timing out.
The User Journey will wait for the HTML document to be completely loaded and parsed, but without waiting for stylesheets, images, and subframes to finish loading. The browser will wait for the DOMContentLoaded event. This is recommended for any website experiencing any issues with slow loading resources, such as slow third-party plugins that you are happy to ignore. The Journey would still fail if it was unable to complete any of the actions.
The User Journey only waits until the HTML document is downloaded. This is generally not recommended as the Journey will attempt to continue before any resources (such as images) have been downloaded.