[Affected versions]: - Firefox 59.0a1 Build ID 20171228220110 [Affected Platforms]: - All Windows - All Mac [Prerequisites]: - Have enough history so that all 12 Top Sites are populated. [Steps to reproduce]: 1. Start the browser with the profile from prerequisites. 2. Quickly drag a Top Site from the 1st row over the ones from the 2nd row. 3. Observe the behavior. [Expected results]: - The preview appears as soon as the cursor is over the Top Sites. [Actual results]: - The preview lags behind a few Top Sites. [Notes]: - The issue is not reproducible if the Top Site is dragged over the same row it came from. - The issue is also reproducible if a Top Site is dragged from the 2nd row over the 1st. - Attached a screen recording of the issue.

Ha. I wonder if this is just javascript processing all the recursion when dragging from further away. You can somewhat see the delay in the video when dragging the last item to the first spot of the second row. We can easily get tens if not hundreds of dragover events. One simple short circuit would probably be to not recompute if the dragover position is the same as the last computed dragover.

Added my recording of drag drop in today's Nightly on reference hardware. The performance seems to have improved. Can you still reproduce? Thanks.

Seems like bug 1428488 also fixed the slowness. Closing as worksforme.

I'll guess it was bug 1428727 as it replaced the recursion with some loops.