A test case like this does not benefit from the async image decoding. <div style="background-image: url('image.png');"></div> Therefore, the decoding happens on the main thread which may affect the scrolling thread.
<rdar://problem/31024112>
This already covered by the patch of https://bugs.webkit.org/show_bug.cgi?id=169547 *** This bug has been marked as a duplicate of bug 169547 ***