this study investigated the effects of manipulating the cognitive complexity of tasks along +/- few elements and the time limits on l2 learners writing performance. to conduct the study, 60 iranian efl learners with two levels of proficiency, low and high intermediate, were selected and assigned to three groups based on the time devoted to task completion. the participants performed both a simple and complex version of information-gap, opinion-gap, and reasoning-gap tasks. the learners writing performance was measured in terms of accuracy, syntactic complexity, and fluency. the results indicated that increasing cognitive complexity of the tasks positively influenced the accuracy of the reasoning-gap tasks but not the information-gap and opinion-gap tasks. regarding syntactic complexity, more complex sentences were measured in the more complex information-gap and reasoning-gap tasks. the increase in the cognitive complexity of the tasks enhanced the fluency measure regarding the information-gap and reasoning-gap tasks. therefore, the findings partially provided support for robinsons cognition hypothesis. with regard to the time limits, the results did not show the significant influence of the time limits on writing performance in terms of accuracy and syntactic complexity. furthermore, there was no significant difference between the low and high intermediate learners regarding accuracy and syntactic complexity. the findings of the present study have practical implications for syllabus designers, english language teachers, and testers.