When using external page conversion actions, a conversion should be counted when users access the external page. However, Nelio A/B Testing has no access to pages outside WordPress, which means it has to count a conversion when a user clicks on the link that will take the user to that page. Whenever such a click occurs, Nelio A/B Testing will detect it and send the tracking information to Nelio's servers (which takes a few miliseconds).
If you want the website to be as responsive as possible, select Force New Tab. This way, links will be opened in a new tab and the synchronization process will occur in the original tab. If, on the other hand, you don't want pages to be opened in new tabs, select Do Not Force New Tab.