Summary

In a workflow with ~200 nodes, panning the canvas at a low zoom level (where many nodes are visible at once) drops to ~14-24 fps with worst frames over 170 ms. Chrome DevTools Performance recording shows that imageCaption.js:1956 updatePosition accounts for ~62% of frame time (3921 ms out of 6000 ms), and the resulting browser Recalculate Style jumps from baseline ~14% to ~68% of frame time.

Disabling ComfyUI-Prompt-Assistant entirely restores normal panning performance.

Root cause (from reading the source)

In js/modules/imageCaption.js around line 2027, every per-node assistant wraps app.canvas.onDrawBackground:

const originalDrawBackground = app.canvas.onDrawBackground;
const onDrawWrapper = function () {
    const ret = originalDrawBackground?.apply(this, arguments);
    updatePosition();
    return ret;
};
app.canvas.onDrawBackground = onDrawWrapper;

This means:

1. The wrap chain grows linearly with the number of attached assistants. With N assistants, every canvas frame walks N nested function calls and runs N updatePosition()s.
2. updatePosition() (line 1956) writes to containerDiv.style.left/bottom/transform every call → forces browser layout / Recalculate Style on N elements per frame.
3. There is no visible-node / viewport culling — assistants update position even when their owning node is far outside the viewport.
4. The cleanup function (line 2036) only restores onDrawBackground if it currently equals the wrapper — so once another wrapper is added on top, that assistant can never be unwrapped, and the chain grows monotonically.

The combination produces O(N²)-ish-feeling behavior at high N: every frame walks every wrap, even for invisible nodes.

Reproduction

1. Load a workflow with 100+ valid nodes for ImageCaption attachment
2. Zoom canvas out so most/all nodes are in viewport (e.g. app.canvas.ds.scale ≈ 0.05)
3. Pan the canvas continuously
4. Observe FPS in DevTools Performance / dropping below 20 fps

Suggested fixes

• Viewport culling: in updatePosition, early-return if assistant.node.getBounding() does not intersect the visible canvas area
• Single global tick instead of per-assistant wrap: register one onDrawBackground wrap that iterates ImageCaption.instances once per frame, instead of N nested wraps
• Skip update when transform/position has not changed: cache last known values and only write to DOM on change
• Throttle: at very low zoom, drop the update rate to e.g. 10 fps via EventManager.debounce

The single-global-tick change alone should give a dramatic improvement.

Environment

• ComfyUI frontend 1.42.x
• Workflow: ~200 nodes, mix of standard + custom
• ComfyUI-Prompt-Assistant: latest at time of testing

Discovered while developing ComfyUI Mirror Panel, where the perf hit was initially misattributed to the Mirror plugin until DevTools profiling pointed at imageCaption.js.