I forced there to be a single decoder thread and could reproduce a similar effect on 70: https://perfht.ml/2NWB77t It is a fairly complicated pipeline, doing color management, downscaling and ADAM7 interpolation. The images are all much larger than they appear (say 500x500) which contributes to the problem.
Bug 1591030 Comment 6 Edit History
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I forced there to be a single decoder thread and could reproduce a similar effect on 70: https://perfht.ml/2NWB77t It is a fairly complicated pipeline, doing color management, downscaling and ADAM7 interpolation. The images are all much larger than they appear (say 500x500) which contributes to the problem. Similar trace on 72 / nightly, which includes the premultiplication SSE2 improvements: https://perfht.ml/34JvYGq