The notification that pops up on the top right of the CRM when someone has added contacts to a Campaign Activity can sometimes pop up too early i.e. when not all the contacts have actually been added yet. Maybe adding a notification saying that 'we'll let you know when it's ready' or a progress bar would be better until they have actually all filtered through and added to the activity successfully. Most customers will have relatively large lists of targets so this popping up early implies it's done instantaneously when it's actually not. https://www.useloom.com/share/2974b986571b44c780f63a4a615ebf07
The contacts are currently added via a background process but your suggestion for better user feedback is a good one. We will look at improving this in a future version, most likely with a notification when the contacts have been added rather than a scrollbar.