It's amazing bugs like this persist, even with the significant resources of several megacorps allocating legions of full-time developers to the codebase.
Their incentive is to keep the browsers good enough to not lose market share. Other than that, the incentive is to either close the web down, or to make the experience as shitty as possible without leading users to switch away, so they can steer users towards the more closed-down native apps.
Unfortunately, companies have an incentive to put us into walled gardens, so the only company that actually cares about the web is the company whose only business model is selling a browser.
Summary: In certain circumstances webview rendering is slow because it has to integrate with the android rendering API incase an app wishes to synchronize some App UI with web UI.
This isn't a limitation in chrome itself, which means chrome rendering has better performance for these specific operations (animations mostly).
Personally I think the chrome rendering path should always be used, and apps which want to draw under/over web content will have to deal with a frame or two of delay.
My understanding of the system webview is that applications can share the resource instead of each using their own copy, reducing memory usage and improving multi-tasking. Facebook's solution seems to side step that benefit. I use the fb browser much less frequently than my normal browser, and id rather not use the fb browser at all. I imagine this is true for most users, so the security benefit seems overstated.
Unfortunately, companies have an incentive to put us into walled gardens, so the only company that actually cares about the web is the company whose only business model is selling a browser.
This isn't a limitation in chrome itself, which means chrome rendering has better performance for these specific operations (animations mostly).
Personally I think the chrome rendering path should always be used, and apps which want to draw under/over web content will have to deal with a frame or two of delay.
Basically, FB's engineering replaced it with a full chromium based webview and explained the reasons android webview sucks ass.