It is a little bit apples and oranges. This is a simple point-in-plane index, PostGIS is a full-blown geospatial data modeling capability. PostGIS is going to be much slower but it is also doing much more as part of the process so that it is useful for sophisticated spatial applications. PostGIS is not fast relative to comparable systems but this particular comparison is not fair because PostGIS is expected to do so much more.

That said, I have also implemented large-scale point-in-plane indexing systems, though not in a while. Per CPU core, you can encode and index around 20M geo-points per second if the implementation is good.