In this paper, we report a longitudinal study of the effects of procedural task repetition on learners' task performance (i.e., syntactic complexity in relation to lexical complexity). We investigated how task repetition results in differences at the group and individual level across each task interval (T = 7). Intermediate-level Saudi learners of English (N = 93) performed a written task biweekly over the course of a whole semester. To control for text type, mode and tenor were fixed at each data elicitation moment, but field was varied in these writing tasks to keep learners engaged. Using Bayesian generalized additive mixed models and mixed location-scale models, we analyzed specific ways that groups and individuals changed across time over the course of each task iteration and in relation to previous points. Our results showed that learners' task performance demonstrated nonlinearity, stability, and variability with meaningfully different effects on individual and group-level development over time.