Root cause is the frame scheduler in cached_network_image_ce (same flaw as flutter/flutter#24804): every frame is rounded up to the next vsync, so 20ms emote frames play at 60% speed on 60Hz and 80% on 120Hz; fix submitted upstream in Erengun/flutter_cached_network_image_ce#79 and verified on a moto g54 5G and a Redmi Note 12 Pro 5G (87 to 89% speed before, 98% after).
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Comment by @M0n7y5