Nexus 6P may have had its launch back in 2015, but the users have discovered the bootloop issue only recently. Bootloop is when the device fully boots up and starts over in constant looping. The issue seems to have been extensively bothering the users for quite some time.
XCnathan32, an XDA user, being assisted by two other users, have been able to create the fix and made it available for everyone else. It is apparent that the four performance cores of Snapdragon 810 of Nexus 6P prevent the device from proper bootup. The fix shuts down all of the four cores.
To be further specific, the A53 cores are left alone, while the A57 cores are disabled. Therefore, users are to notice a little improvement of battery life during lags. It is so because the A53 cores are more power-efficient than the A57 ones.
However, in your usage of the fix, you require using the device with a bit of technical knowledge. The fix includes flashing various modified images and, unlocking the bootloader of the device. The users are advised to know more about these two points at least so that they can get the best out of the device while using the fix.
This is also going to help you discover more previously unknown aspects of the device. Considering that, it is important to remember that not everyone yet has had worthwhile experience using the fix.
Fascinatingly, the XDA user, who created the fix, has created a fix for the same issue in Nexus 5X as well. Although, it has not yet been tested by the user, which is why it is yet to be recommendable as of now.
Footnote on Nexus
Both Nexus 6P and Nexus 5X have lawsuits against Google, LG, and Huawei over the concerning issues. Having knowledge on these will further help you be decisive regarding the bootloop issues.