Chrome UI is inflated early on during startup on the critical path (during Activity.onCreate) and takes a long time to do so (200ms). The toolbar is worst offender and most of it is either not visible or not critical and could be done at a later point during startup. WebApkActivity shows a splashscreen while the webpage is loaded so chrome UI does not need to be shown early. 

Design doc: https://docs.google.com/document/d/1ik4yn8nxCoNe4rUAceBuwHtNRknFkCWSVQaR1FGcVcs/edit