The modern media controls employ a backdrop filter for visual effect. In media tests displaying the controls, this backdrop filter is rendered with poor performance in software rasterization mode.

This can cause sporadic and repeatable layout test timeouts as well as WebProcess terminations during the process of resetting the testing environment to default values. Specifically, while about:blank is being loaded, there still might be a rendering phase occurring, handling the filter, delaying the about:blank load and causing WebKitTestRunner to time out on that load and just terminate the process.

Until this gets resolved, I'll attempt to mark the related tests as slow, which should hopefully allow more time for rendering to resolve, avoiding these timeouts.