Several researchers have advocated explicit instruction of vocabulary in order to help students improve their reading comprehension, especially low-achieving readers who need to "catch-up" to their age peers. Very few studies, however, have attempted to compare the time efficiency of direct instruction to its alternatives. In this review, I calculate the efficiency of vocabulary instruction in 14 studies taken from a recent research review (Wright & Cervetti, 2017). I then compare those results with estimates of vocabulary acquisition via a likely alternative source of vocabulary growth, free reading. Free reading was found to be 1.7 times more efficient than direct instruction in building vocabulary in short-term treatments, and 12 times as efficient for long-term treatments.