Looks like the Android 8.1 update brought along a new issue with it. Over at the official Google Pixel community, a lot of users have been complaining about slower screen wakeup response for more than a week now.
Specifically, users - majority of whom are Pixel 2 XL users - are complaining that post the update, whenever they try unlocking their device through the fingerprint scanner, the device's screen wakes up after a significant delay.
Some say they observe the same behavior on pressing the power button as well. Turning on the 'always on display' seems to have fixed the problem for some of those who tried this workaround.
The issue has been escalated to the concerned team, and a Google employee recently said they'll be reaching out for bug reports.
My One Plus 3T is the same using the fingerprint reader to wake the phone since upgrading to Oreo
this is probably the worst phone ever allowed to leave the factory. Whoever was behind the quality control department should seriously be replaced with better management