Botond's suggested fix hugely improves things, but the first frame of the zoom is still a bit slow. That should be fixed by bug 1598380.
Bug 1599248 Comment 9 Edit History
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Botond's suggested fix hugely improves things, but the first frame of the zoom is still a bit slow. That is due to bug 1598380. So let's make this bug about fixing double-tap-to-zoom to work like pinch-zooming, and deal with the remaining slowness in the other bug.