When they get closer to having the OS WebView actually updated via Chrome channels then it's conceivable that the apparent startup time will come down, and it wouldn't surprise me if this is one of the main reasons for it.
If you see this recent thread: https://groups.google.com/a/chromium.org/forum/#!topic/blink...
They even say this: "The web is quite far behind in mobile, which is why we're applying a greater amount of focus to solving the problems we have on mobile, even at the expense of nice-to-have features."
This is quite an admission, and at odds with a lot of their noise. It's also telling about their problems because Chrome suffers from a sort of death by a thousand cuts. On mobile to get the user experience people demand (they aren't wrong to be after 60fps performance as studies have shown mobile users are even pickier than desktop net users) you simply can't afford things like that anywhere, let alone right as the app starts up.