Speed reading, when used in an effective manner, can allow you to cover huge amounts of information with high recall in a much shorter time than the average reader, however being able to run through great amounts of material does have its shortcomings. There a few situations where speed reading is not the most effective way to learn and they should be noted.
Foreign Language - Obviously if you are unable to understand something normally, you have no chance of understanding it at a faster rate. If you do have a grasp of the language and can read in that language at a normal rate, then speed reading should not be a problem. The problem arises when there is a lack of understanding of the proper translations that can make you not able to speed read it. Common sense, if you can't read it normally you most likely can't read through it at a great speed.
Math Concepts - Math Formulas are meant to help you understand the relationship between different numbers and variables. Of course you can read quickly over the texts that accompany the formulas, but it is best to slow down and learn those at a slower speed
Poetry - After taking many years of English classes, it turns out to get the full effect of poetry, it can't be rushed. Poets often write their words with hidden meanings that need extrapolations and interpretations, or something like that. Poetry takes time to be absorbed, so to get the full enjoyment and experience should not be read at your accelerated rate.
Beyond Your Limits - Anything from molecular biology, to advanced physics, to micro-engineering, if you only understand half of the words on a page, it shouldn't be read at the fast rate. While you will pick up some of the information , you will most likely be missing out on a few key elements. Speed reading works best when your reading material is in a language you can understand.