A quick look at eye-movements in reading Arabic text with and without diacritics
The two videos below are generated from eye-movement data I recently collected in a short test run for a research application. They show how the gaze moves across the screen as a native Arabic speaker reads text without diacritics (Text 1) and with diacritics (Text 2), or so called unvowelled and vowelled text.
For skilled readers, reading text with diacritics is slightly slower than reading text without diacritics. While this is not a scientific test if this principle, you can see here that the gaze moves faster across the lines in the undiacritized Text 1. In fact, when first encountering a word, this reader looks at it on average 465 ms in Text 1 and 519 ms in Text 2. Note that only content words are diacritized in Text 2, so that this difference would probably be larger if Text 2 was completely diacritized.
Note also how the eyes move in quick, stepwise motions, even though when we read we experience it as our eyes moving smoothly and evenly across the text. (For more details on this, see this graphic.)
Text 1: without diacritics
Text 2: with diacritics