Google looking into reports of slower Pixel 2 XL fingerprint scanner after Android 8.1 update
Apparently, there is a workaround for the sluggish biometric reader. Turning on the always on feature will allow the fingerprint scanner on the Pixel 2 XL to recover its previous speed. The problem with this option is that you might find your battery draining a little faster.
The good news is that Google is working in a fix. The company is aware of the issue and plan on collecting bug reports to see exactly what it needs to send in the way of a patch, in order to fix the issue.
source: GoogleSupport via AndroidCentral