Multiplying Dense matrix by Sparse matrix super slow

[jellezult]

I found that the order of multiplying a dense matrix by a sparse matix really matters:

• Sparse * Sparse -> Pretty much instant
• Sparse * Dense -> Very fast
• Dense * Dense -> Still fast
• Dense * Sparse -> Super slow, scales horribly with matrix size

These are the numbers I got for multiplying two identity matrices of the size shown on the x-axis (e.g. 200x200)

Any idea if this is to be expected, or if there might be a mistake in the mathnet implementation?