One reason may be that Amazon is trying to do too many things with their fire tablets. They're competing with the iPad, which is a very powerful device, as well as lower-priced Android tablets. Another reason may be that Amazon is using a forked version of Android, which is not as popular or developed as the standard Android OS. This can lead to slower development and fewer updates for apps and the operating system. Lastly, it's possible that Amazon is intentionally making their devices slower in order to encourage people to buy more expensive devices like the Kindle Fire HDX.