Relationships among task characteristics, L2 performance, and interlanguage development are of interest both for SLA research and the design of syllabuses and language teaching materials. Complexity has been identified as a promising, but methodologically problematic, task design feature. A study was conducted of the effects of progressive increases in the complexity (operationalized as number of elements) of three versions of each of three tasks on the syntactic complexity and lexical diversity of the speech of 42 English native speakers. Data on native speaker performance are important because they reveal task complexity effects unfiltered by non-native competence. Independent evidence that greater task complexity increased cognitive load was shown by participant self-ratings of perceived difficulty, mental effort, and stress, shorter prospective duration estimates and, using dual-task methodology, slower reaction times. Mid-complex versions of the three tasks elicited the most complex syntactic structures, and the most complex versions elicited the greatest lexical diversity. Implications are noted for the design of parallel studies with non-native speakers, along with suggested methodological improvements for future research with native and non-native populations.