There are many things that make a smartphone OS good to use, such as its user interface, overall design, features, accessibility, conveniences, and of course speed, which while can be hardware dependent, has also got to do with optimizing the software for hardware. This is something that Google will be addressing with Android O.
During I/O 2017, Google announced that one of the changes that users can expect with Android O is that reboot times for the phone will be cut in half as it will be apparently twice as fast as its predecessor, Android 7.0 Nougat. Google did not dive into specifics as to what they have done to make the changes, but Android O is supposed to be faster.
Google used Sheets as an example, where they said that with Android O, Sheets was found to run twice as quickly. These speed improvements are said to be applied across the operating system, so it will be interesting to see how this will work out in real-life. Now Android O is only scheduled to be released towards the later part of the year, and depending on your phone, some users might only see the update in 2018.
However Google has opened up the Android O beta to the public, so for those who are interested in taking the beta version of the OS for a spin, you can go ahead and do so, but note that since this is a beta, some features might not be working correctly or it might change in the final build.