Ok I think I've done it but need further testing ...
Only possible on rooted devices ..
Go to folder
/data/data/com.android.faceunlock/shared_prefs/
in your root enabled file browser (I used es file explorer)
Open the file inside
Edit delay number to 1
Save file
Then change permissions to read only by all
And ownership to root
Go 1 level up and do the same with the shared_prefs folder (perm and owner)
And again 1 up and same settings to the com.android.faceunlock
Come back here and report if it improved
Not sure if all the permissions must be like that but I went safe on it since file got replaced first few times I tried
Please do.come back here to report if it worked for you too
My delay was 1730 after changing to 1 it worked like charm once but then it changed to ~700(file got updated again) then I've changed the settings of files and folders to present override and now I did not experience the lag yet
Let me know if you need more info or more detailed instructions 0,O