The current study examined the extent to which first language (L1) utterance fluency measures can predict second language (L2) fluency and how L2 proficiency moderates the relationship between L1 and L2 fluency. A total of 104 Japanese-speaking learners of English completed different argumentative speech tasks in their L1 and L2. Their speaking performance was analysed using measures of speed, breakdown, and repair fluency. L2 proficiency was operationalised as cognitive fluency. Two factor scores of cognitive fluency--linguistic resources and processing speed--were computed based on performance in a set of linguistic knowledge tests capturing vocabulary knowledge, morphosyntactic processing, and articulatory skills. A series of generalised linear mixed-effects models revealed small-to-moderate effect sizes for the predictive power of L1 utterance fluency measures on their L2 counterparts. Moderator effects of L2 proficiency were found only in speed fluency measures. The relationship between L1 and L2 speed fluency was weaker for L2 learners with wider L2 linguistic resources. Conversely, for those with faster L2 processing speed, the L1-L2 link tended to be stronger. These findings indicate that the L1-L2 fluency link is subject to the complex interplay of phonological differences between learners' L1 and L2 and their L2 proficiency, offering implications for diagnostic speaking assessment.