Speedreading comes with the compelling promise that you can read [insert some number from 5 to 100] times faster with good comprehension. But this doesn’t make any sense—understanding requires cognitive processing that takes time and effort. This sets an upper bound of your learning speed and there is no way to get around it.
Think about it this way: if we can read anything five times faster without losing understanding, then it will also be possible for us to think five times faster. Neither is actually possible, our cognitive capacity caps how much can happen at a time.
It may be possible to remember the book content and attain a very superficial understanding with speedreading. This is especially likely when it’s coupled with mnemonics. But the level of understanding resulted is inadequate in most learning contexts. They certainly won’t make you capable of applying the knowledge flexibly to solve problems, otherwise the top experts in every domain will all be speedreaders.