"Reading" means a number of different things, a problem that needs to be be addressed before questions of Hangul vs. English can be addressed. At the most basic level, it refers to the ability to perceive written material and articulate its content. To even talk about a "rate", you have to settle on comparable units. There are no comparable units for perception of written language that can be applied to English and Chinese or Korean. You might count segmental phonemes (if we could agree on the phonemic analysis of the languages, which we cannot), or you count count syllables (if we could agree on the number of syllables in certain words of English, which we cannot). Invoking stress feet would be a better measure for English, but not Chinese or Korean. The count of words would favor English, unless you redefine "word" so that various grammatical words of English are deemed to be part of some other word. Korean words are generally longer than English words. Chinese words are segmentally and syllabically shorter than English words, on average, but Chinese syllables are durationally longer than English syllables, so it matters very much what thing you are measuring. Actually, Hangul and Chinese do not have roughly one character per syllable, since Hangul is more a syllable-structures alphabetic system, see for example [pap] 밥 "rice" with the same-ish character in the northwest and bottom positions and "a" to the Northeast. Chinese 休 and 湖 can be treated as respectively 1 character or 2, or 1 or 3, depending on how you count.
The authors don't take into consideration a well-known cultural difference regarding speed-reading and comprehension, which is that experiential background with timed read-and-respond tests (e.g. the GRE) improves performance on these exams. Certain international students have little experience with these types of tests, while others have substantial experience. This is a further variable, one not controlled for, that can influence reading speed and comprehension, in a way that is not related to language.