Another huge bottleneck seems to be off-screen rendering itself: chromiumembedded / cef / wiki / GeneralUsage — Bitbucket
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With off-screen rendering CEF does not create a native browser window. Instead, CEF provides the host application with invalidated regions and a pixel buffer and the host application notifies CEF of mouse, keyboard and focus events. Off-screen rendering does not currently support accelerated compositing so performance may suffer as compared to a windowed browser.