The purpose of this study is to 1) identify whether three speech rates (SR) (normal, faster and slower) have different effects on high-intermediate ESL learners' listening comprehension of academic lectures, and if so, to determine which SR is the most beneficial to the learners' comprehension; and 2) to identify learners' perception of SRs and reasons for lecture incomprehensibility. Eleven international students were asked to listen to three long academic lectures on unfamiliar topics and answer multiple choice comprehension questions, which was followed by a brief survey of their perception of the SR appropriateness and lecture difficulty. The original SRs of the lectures, 157 wpm/3.40 sps, 168 wpm/3.34 sps and 173 wpm/3.33 sps, were set as the normal SRs, which were compressed and expanded with sound editing software by 15% into the faster SRs (181 wpm/3.91 sps, 193 wpm/3.84 sps and 199 wpm/3.83 sps) and the slower SRs (134 wpm/2.89 sps, 143 wpm/2.83 sps and 147 wpm/2.83 sps). Comprehension scores revealed no significant differences in the mean scores obtained at these SRs, which tentatively suggests that neither an ideal SR nor a "threshold" SR exists within the SR range between 134 wpm/2.89 sps and 199 wpm/3.83 sps. At a close-to-significant level, the higher scores obtained at the slower SR, a rate preferred by the subjects, were taken to suggest that the most facilitative SR is likely to be lower than 134 wpm/2.89 sps. Topical and lexical unfamiliarity and SR were reported by the subjects as the major factors affecting lecture comprehension. The non-significant findings were interpreted as being attributed to the low modification rate, the small n and the low reliability of the comprehension questions. Despite its inconclusive results, the study offers both theoretical and practical implications. It highlights the importance of SR in L2 listening comprehension research, where spoken materials rich in acoustic features are recommended to be used as input. Besides informing L2 teachers of important factors that affect listening comprehension, it also raises lecturers' awareness of slowing down for L2 learners.