The Pixel 2 XL is a divisive piece of tech. Many feel it is the best Android device out there and argue that it has the best camera on any smartphone, but there's also been apprehension because of the POLED display, burn in issues. Software has been another point of concern as it seems the Android 8.1 update, which unlocks all those cool AR Stickers, has been responsible for slowing down the fingerprint sensor.
Enthusiasts on Google’s support forums as well as Reddit are chiming in and saying that since the update, the fingerprint sensor has been slowing down or is less responsive. The issue is widespread enought that Google is acknowledging the bug and promises to look into it.
Source: 9to5Google