Understanding when and why nudges work is crucial for designing interventions that consistently and reliably change behaviour. While behavioural nudges have shown promise across various domains, their generalizability, scalability, and transferability remain contested. Recent evidence highlights the importance of addressing unexplained heterogeneity in effect sizes and contextual factors to unlock the full potential of these interventions. This paper explores the relationship between decision-making speed and the effectiveness of two nudges – carbon footprint labelling and menu repositioning – aimed at encouraging climate-friendly food choices. Building on Kahneman’s dual-process theory of decision-making, we examine whether these interventions are more effective in fast, intuitive (System 1) contexts compared to reflective, deliberate (System 2) ones. Using an incentivized online randomized controlled trial with a quasi-representative sample of British consumers (N=3,052) ordering meals through an experimental food-delivery platform, we introduced a time-pressure mechanism to capture both fast and slow decision-making processes. Our findings suggest that menu repositioning is an effective tool for promoting climate-friendly choices when decisions are made quickly, though the effect fades with extended deliberation. Carbon labels, in contrast, showed minimal impact overall but reduced emissions among highly educated, climate-conscious individuals under time pressure. The results imply that choice architects should apply both interventions in contexts where consumers make rapid decisions, such as digital platforms, to help mitigate climate externalities.