Lots of things to try - swap to the 4K Cache (recommended anyway); if that doesn't help leave it on the 4K cache, and try precalculating the cache and then loading that rather than calculating it afresh for each frame (
); if that doesn't work try raising the precision; if all else fails, using Path Tracing in place of a Cache should work though will be slower to render. Please do let us know which solution ended up working too, thanks!