Actually I had to do it twice because the first time it installed a system update, but even then it only took 26 seconds to install, finish booting, and show the lock screen. System updates have been seriously optimized in the last few Android releases. They used to take like 5 minutes.

On the Pixel phones Google uses two system partitions. One is the one you're currently running and the other one gets the update applied to it in the background without you having to stop using your phone. When you reboot it just has to flip which system partition it boots from and maybe do some first boot actions. If the phone starts up successfully it'll then apply the update to the original partition to keep them in sync.

My iPhone 8 was 20s to login screen. It basically instant once I login but that may be because my password takes a while to type and it’s loading the rest while I type.