1. Go to http://www.blobsallad.se
2. Open Canvas tab in Web Inspector
3. Press the record button for the Blob canvas
4. Wait for two seconds
5. Press the stop button
It took over a minute to show the recording. "Processing recording" stayed on for over a minute.
The recording shows in a few seconds. If that isn't possible, it should show the recording with limited features at first.
This is most likely caused by <https://webkit.org/b/175282>. There is no "easy" way to quickly determine if a particular visual action caused some pixels to change on the canvas, so we have to resort to checking every single pixel. On larger canvases, this can take a long time, and the problem is compounded if there are a large number of visual actions in the recording.
This was addressed in https://bugs.webkit.org/show_bug.cgi?id=185758.