I think Google provides the longest upgrade period among all Android phones (Which clearly is not enough). But the good thing about Nexus/Pixel is that the boot

I think Google provides the longest upgrade period among all Android phones (Which clearly is not enough). But the good thing about Nexus/Pixel is that the bootloader is unlocked (if you buy Pixel from the Google store not Verizon). An unlocked bootloader enables you to flash cutom ROMs on your phone. For snappiness, most of new phones with Android 8.1 are fast enough. But again, if there is a big community for that phone in XDA (like with Nexus phones), and the bootloader is unlocked, you will be able to flash new versions of Android for a long time.