[magicalhippo]

I'm no expert, but my immediate thoughts are that evaluating a gaussian blob is very simple, it's just an exponential of a distance. The edge of a sphere makes it more complicated to compute, hence slower.

For backpropagation, the differentials of a gaussian is smooth while it's not for a sphere, again because of the edge.

Now, if you want to use a sphere you probably will do something like using an opacity falloff similar to ReLU[1], making it transparent at the edge.

This should make smooth enough as such I guess, but I imagine you still have the more complicated rendering. Though I may be mistaken.

[1]: https://en.wikipedia.org/wiki/Rectifier_(neural_networks)