An online system maintains a plurality of content items. The online system selects and provides content items to users of the online system in response to impression opportunities to provide content items to users. A plurality of segments of the impression opportunities are determined. Each segment categorizes the impression opportunities. A relationship between a value metric and computing resources used in the selection process are determined for each segment. Each relationship provides a rate of increase of the value metric given an increase in computing resources used. An allocation of computing resources used per impression opportunity for each of segment is determined based on the rates. A plurality of impression opportunities are identified. In response, one or more content items are selected for each impression opportunity using computing resources according to the determined allocation for the segment to which each impression opportunity belongs.