UnlockManager waits for confirmation that the phone has displayed its "Smart Lock: Chromebook unlocked" notification before finally attempting to unlock or sign in to the device [1]. This is actually after the Chromebook had already received confirmation that it was okay to unlock or sign in to the device -- which means Smart Lock is waiting for an additional roundtrip communication with the phone, which may be unnecessary, and causing increased latency.

There are three steps I anticipate in this investigation:
1) Determine if this extra roundtrip is causing significant latency. If so...
2) Determine if this extra roundtrip needs to occur before trying to unlock or sign in to the device (it could be a security requirement to guarantee the user is informed that Smart Lock was used).
3a) If not necessary, then attempt unlock or sign in as soon as we get the okay from the phone; inform the phone asynchronously.
3b) If necessary, investigate if using Connection Preservation model can cut down on latency (example: [2]).

1) https://cs.chromium.org/chromium/src/chromeos/components/proximity_auth/unlock_manager_impl.cc?type=cs&sq=package:chromium&g=0&l=173
2) https://cs.chromium.org/chromium/src/chromeos/components/tether/connection_preserver.h?q=connectionpreserver&dr=CSs&l=20