1. A computer-implemented method for content item segment optimization, comprising: under the control of one or more computer systems configured with executable instructions, generating a plurality of content item segments for each content item of a plurality of content items available to a plurality of users on a network resource, each of the plurality of content item segments being a portion of the content item; providing one of a plurality of content item segments generated for a content item of the plurality of content items in response to each selection of the content item by one of the plurality of users; tracking user activity on the network resource subsequent to the user accessing the provided content item segment, the user activity being associated with the content item; determining a level of user interest in the provided content item segment based on the recorded user activity; assigning scores to each content item segment provided to the users, each score corresponding to the determined level of user interest in the content item; combining assigned scores for each content item segment provided to the users; in response to determining that two adjacent content item segments in the plurality of content item segments have a respective combined score that is first highest and second highest among the plurality of content item segments, and that the respective combined scores for each of the two adjacent content item segments are within a predetermined margin, shifting a boundary between the two adjacent content item segments resulting in the creation of two modified adjacent content item segments; and selecting one of the two modified adjacent content item segments as a representative of the content item to be provided in response to subsequent requests for the content item.