<p>Language processing in monolingual and heritage speakers shows variation when the concept of aspect processing, an understudied phenomenon in the literature, is taken into consideration. In this regard, this study focuses on grammatical aspect (i.e., imperfective and perfective) in monolingual and heritage speakers of Turkish, and aims to uncover Turkish monolinguals’ and heritage speakers’ processing of perfective aspect for completed events and imperfective aspect for ongoing events. Furthermore, the study also aims to explore whether language proficiency and processing speed capacity have an effect on language processing in heritage speakers’ eye-tracking data. The data have been collected by employing a comprehension experiment with 28 monolingual speakers and 25 heritage speakers. The heritage speakers participated in an additional sociolinguistic survey, a language proficiency tests (i.e., self-rating and C-Test), and a cognitive speed index test (i.e., WAIS-4). The results indicate that both groups demonstrate greater sensitivity to the imperfective aspect than to the perfective aspect. The accuracy rates were higher in the imperfective condition, and the effect emerged earlier in this condition compared to the perfective condition. Additionally, the findings suggest that language proficiency and processing speed significantly influence grammatical aspect processing and comprehension. Specifically, participants who achieved higher scores on the linguistic and cognitive tests exhibited correspondingly higher accuracy rates. To conclude, the study highlights how linguistic structures differentially affect comprehension in monolinguals and heritage speakers, and how individual differences shape heritage speakers’ language processing abilities.</p>