That’s one of the things that happen when you want to get an Android update 5 minutes after its release and still expect a bug-free update.
These two things are very different and most of the times you can’t achieve them both at the same time, one has to be sacrificed.
The Google Pixel 2 XL which recently got its Android 8.1 update just got a bug issue detected. After Android 8.1 update, the phone’s screen takes so long to turn on when unlocked with the fingerprint scanner.
The fingerprint works properly, as you can feel the vibration effect after putting your fingers upon the sensor, to show that the response is very much active, but the screen takes long and delays a little above 2 seconds before turning on.
There again, in some cases, turning on AOD will fix the bug a little, but so many people will prefer to rather leave that off.
Some people have also reported that the delay can also be noticed when the power button is pressed. It takes about a second until the screen turns on.
Therefore, I think the problem isn’t from the fingerprint; it’s actually from the screen’s inability to respond properly to the fingerprint.
Turning on the screen first with the power button and unlocking with fingerprint works instantaneously.
If you are experiencing this same bug on your pixel, you are not alone, and take note that Google is now aware of the bug and they’ve started work to fix it immediately and the fix will be available in a future update.