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Two things could explain the lag: the blur is re-running every frame, or the mask is forcing a repaint. Worth separating them before picking a fix.
Animating mask-position never composites — it repaints the element. And the element being repainted sits inside a displacement chain, so the whole filter re-executes on the CPU each frame. That compounds: it isn't one cost, it's the repaint feeding the filter.
So the fix isn't tuning the blur radius. The animated property has to leave the paint path entirely. If the field is painted statically and covered by a same-coloured curtain, translating the curtain is visually identical to masking — and transform composites.
One trade-off: the waves travel with the band instead of rippling through noise fixed in space. In motion, near-indistinguishable.