Nicholas Carr writes: "Just as the pioneers of hypertext once believed that links would provide a richer learning experience for readers, many educators also assumed that multimedia, or "rich media," as it's sometimes called, would deepen comprehension and strengthen learning. The more inputs, the better. But this assumption, long accepted without much evidence, has also been contradicted by research. The division of attention demanded by multimedia further strains our cognitive abilities, diminishing our learning and weakening our understanding. When it comes to supplying the mind with the stuff of thought, more can be less." (Carr, The Shallows, Kindle, 2,211-23)
Multimedia requires multi-tasking while learning and understanding (deep understanding) requires focus and mono-tasking.