Search task characteristics are recognized as important factors that affect search process and its outcomes. We examine the relationships among operational measures of searcher's behavior, individual cognitive differences, subjective task difficulty and mental effort assessed by dual-task performance. A web-based information study was conduced in a controlled experimental setting. Forty eight study participants performed six search tasks of varying type and structure. Subjective task difficulty was found to be influenced by the searcher's effort measured as the number of result pages and individual documents visited, the number of documents marked as relevant, as well as by individual cognitive differences, and mental effort assessed by performance on the secondary task. In contrast to previous studies, no strong effects of user navigation graph structure were found.