This study investigated the effects of two learner-related factors on English as a first language (L1E) (n = 20) and English as a foreign language (EFL) (n = 20) learners’ incidental vocabulary acquisition through reading an authentic novel. Central to the study was the participants’ reading of the novel: A Clockwork Orange by British novelist Anthony Burgess. The novel contains slovos, which are words from nadsat, a foreignized argot used by the teenage gang members in the novel. Two unannounced tests followed the reading to measure the participants’ word meaning recall and recognition of target words. Regression analyses revealed that reading speed was a robust factor that affected both the L1E and the EFL learners’ acquisition of both word meaning recall and recognition; apparently, the faster the reading, the more words the readers acquired. However, positive affect, measured as reading enjoyment, did not turn out to be a factor in word acquisition. The findings indicate that the cognitive factor, i.e., reading speed, overrides the positive affect of reading enjoyment in determining L1E and EFL learners’ incidental vocabulary acquisition through reading. This research concludes with a discussion of the pedagogical implications of repeated and extensive reading, which promote the development of reading speed.