SLA research and the design of syllabuses and language teaching materials have focused on the links among task features, L2 performance, and interlanguage progress. The present study considers complexity as a promising, but methodologically problematic, task design characteristic, and analyzes the effects of gradual growth in the complexity of three versions of each of three activities on the syntactic complexity and lexical diversity of the speech of 42 English native speakers. These data are relevant in a comparison with non-native competence. The greater the task complexity, the greater the cognitive load and difficulty perceived by participants, the mental effort, and the stress. Instead, prospective duration estimates decreased, and reaction times slowed down. Implications are useful for the design of parallel studies with non-native speakers, and future research with native and non-native populations.