Small fast interpolators:

Intergrid: interpolate in an N-d box grid, uniform or non-uniform.
This is just a wrapper for scipy.ndimage.map_coordinates and numpy.interp.

Barypol: interpolate in a uniform N-d box grid, using d + 1 corners of a simplex
(triangle, tetrahedron ...) around each query point.
From Munos and Moore, "Variable Resolution Discretization in Optimal Control",
1999, 24p; see the pictures and example on pp. 4-5.
It's implemented in a C++ header barypol.h, with a Cython wrapper.

In 4d, 5d, 6d, Intergrid does around 3M, 2M, .8M interpolations / second.
Barypol is ~ 5 times faster, but not as smooth.
(Both will of course become cache-bound for large grids.)
See interpol/test/*.log; ymmv.

Comments are welcome, testcases most welcome.