Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for distributing content items. In one aspect, a method includes receiving user interaction data specifying user interactions with a publisher resource. A heat map specifying levels of user interaction with various portions of the publisher resource is created. Content item slot performance information specifying one or more performance measures for content items presented in various candidate content item slot locations are identified. One of the candidate content item slot locations is selected based on the heat map and the one or more performance measures. Data that cause presentation of suggestion information that identify the one candidate content item slot location as a suggested content item slot location are generated and output.