Students were given text to read, with 40\% of words longer than 4 characters had their letters jumbled. Letters were shuffled at the front, middle and end of words. Whereas normal reading speed is 255 words per minute, when letters where jumbled in the middle of a word, the rate dropped to 227, dropping even further to 189 when letters were jumbled at the end. When letters were shuffled at the start of the sentence, performance dropped to 163. Participants were also asked comprehension questions and performed pretty well, although some noted there were some unknown words.
Letter transpositions (as the cool kids call it) that cross morpheme boundaries have also been shown to incur a penalty in another work of the authors. When letters are substituted instead of transposed, performance unsurprisingly drops a lot as well.
Keith Rayner et al. \Raeding Wrods With Jubmled Lettres: There Is a Cost”. In: Psychological science 17 (Apr. 2006), pp. 192{3. doi: 10.1111/j.1467-9280.2006.01684.x.