On a fairly simple page, capturing a trace with screenshots while scrolling can end up with some very heavy GPU work.

Developers (like prashantpalikhe and samccone) have ended up seeing huge green GPU bars that end up throttling the rendering system back to 30fps and otherwise making it difficult to understand why things are so slow.

If we could land some trace events to capture the time spent in gpu readback, then we could share with the user that significant time in this frame was spent capturing screenshots (and that they might want to retry with the checkbox off).