Quick answer: Godot 4 AnimationPlayer onion skin freezing the editor on complex scenes? Onion skin replays the full subtree per frame - disable it for the heavy animation, not globally.
Scrubbing a 4s walk-cycle animation in the editor locks the UI for a beat each frame. Profiling shows the onion skin pre-render is the cost.
Disable per-animation
In the AnimationPlayer dock, toggle the onion skin button off for the active animation. The global preference re-enables it next time you open a different track, but the per-clip state sticks.
Reduce skin frame count
Drop Onion Skin Steps from the default 4 to 1. Each step is a full re-render of the subtree - 4 steps is 4x the cost for marginal artistic benefit.
Cull invisible nodes
Mark mesh nodes you don't need to see in the preview as visible = false for the editor pass. The onion skin honors visibility.
“Onion skin is a great feature; it's also a great way to make a 60fps editor a 4fps editor.”
Keep a dedicated camera node positioned for animation preview. The editor camera moves around; the preview camera doesn't, and onion skin is happier with a stable view.