Speed reading is good for familiarity with the subject you are reading. It isn't going to make you an expert in the material you are reading. Employed correctly it can be a useful technique exposures, but it isn't to get you mastery of a subject.
For example;
In medical school we have to read a lot of material, understand said material and be sufficient in recall of that material for critical thinking/clinical reasoning problem solving type test questions. This makes wrote memorization a poor strategy for doing well on most tests in medical school (obviously exceptions like gross anatomy exist, which for the most part is just memorization).
I use speed reading in my technique to assimilate large amounts of information--I do it as prereading before my lectures. Then use active note taking during lecture, then the same day do "reading for understanding" which is more an active-thinking type of reading done much more slowly where I put together processes in my head as I read and make notes of it.
If you really, really want to understand a new subject speed reading isn't the way to go. Its exposures, most people need 5-6 exposures to really start to comprehend a new subject/idea.