Yeah based on that, the workaround shouldn’t be needed, but it definitely will accomplish it if you can’t get past the bottleneck.
If the fan is spinning fast, I’d expect either the light function or the real shadow to get blurry since neither method generates motion vectors.
The only drawback I found was that the light function was perfectly sharp regardless of distance, hence the need for a distance field alpha or some other way to control the penumbra. A lazy alternative would be to use the mipmap to blur the texture based on distance.
I found the light function to have less noise, since it is not stochastic.