This study investigates the intriguing scenario where L2 learners can outpace their L1 speech rate. Prior research indicates a faster speech rate in Japanese compared to Mandarin. However, the question remains whether native Mandarin learners can overcome their inherently slower L1 speech rate when speaking L2 Japanese. We assessed 15 N1-certified Mandarin learners of Japanese, divided by immersion experience—seven with at least a year in Japan, and eight without immersion. Their speech rates in both languages were measured against those of ten native speakers per language, including reading and spontaneous speech. Challenging the L1 superiority belief, our findings reveal that the immersed group could match the faster speech rates of native Japanese speakers, thereby exceeding the speech rates of their L1 Mandarin, which are similar to other native Mandarin speakers. Conversely, the non-immersed group’s Japanese speech rate was comparable to or slower than their L1 Mandarin. Subsequent analysis probed how speech rate correlated with other learner variables such as gender, age, study duration of Japanese, and length of residency in Japan. The findings also highlight immersion as the critical factor of speech rate. This study extends our knowledge of bilingual fluency, providing new perspectives on L2 prosody mastery.