Can someone explain the unload option at sop level.
I'm trying it in Houdini 8.0.342 and it seems to be doing the opposite
of what I'm expecting it to, namely speed up the network.

I thought the idea of the unload option was to clear out memory which probably won't speed up the network because it might well need to reload that memory for the next cook.

That is right, unload <SOP> will free all the memory of the above given SOP. As long it is not needed, use unload, otherwise if the network needs some data from the above sop, it will recook it.